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› Restoration of an Eko Viscount Reverb amplifier

Restoration of an Eko Viscount Reverb amplifier

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Stefano267
Inscrit le: 16 Oct 08
Localisation: Autre

EKO VISCOUNT REVERB SCHEMATIC DIAGRAM

Hello guys,
I’m back again and this time I post the schematic diagram with some notes. For simpler reading I’ve divided the schematic in four sections:
- Power supply
- Channel 1 (Canale 1)
- Channel 2 (Canale 2)
- Power amplifier

The electronics of the Eko Viscount Reverb are not complicated apart from channel 1 that has also the vibrato and the reverb effects. As you know by now I had a hard time with the vibrato stage, because in my amplifier both controls (speed and intensity) were missing, so I had to figure out the original wiring, looking at the component board, and I chose the component values looking at other amplifiers schematic diagrams and making some simulations.

Tube naming convention
I named the tubes starting from the first EL84 (V1) on one side going to the opposite one where the ECL86 (V10) is placed. Here are the tubes, the names used in the schematic diagram and their function in the amplifier.
V1 – EL84 – power output
V2 – EL84 – power output
V3 – EL84 – power output
V4 – EL84 – power output
V5 – ECC83 – phase inverter and driver
V6 – ECC83 – low frequency oscillator and neon bulb driver (vibrato effect)
V7 – ECC83 – channel 1 second and third preamp
V8 – ECC83 – channel 2 first and second preamp
V9 – ECC83 – channel 1 first preamp; reverb first preamp
V10 – ECL 86 – reverb second preamp (triode); reverberation unit driver (pentode)

Power supply

Let’s begin with the simplest stage, the power supply. The mains power goes to the primary winding of the power transformer through a voltage selector and, of course, through the mains switch. The fuse is installed inside the knob of the voltage selector on the front panel. As you can see there is also a capacitor between the output of the mains switch and a pin installed on the chassis. This, if too much hum noise is experienced, can be connected to the chassis (ground). In many amplifiers of the 60’s this was a common solution to hum noise reduction, but actual safety regulations prohibit this kind of workaround. In such a way the chassis is partially connected to mains, but the chassis is also the signal ground and so also everything connected to the amplifier will be partially connected to mains: instruments and other electronic equipment. If that capacitor fails (i.e. short-circuits), ground is DIRECTLY connected to mains and so all the rest. VERY UNSAFE!!! I will definitely remove that capacitor and I will connect the chassis to ground (earth). Ground connection should remove all hum noise.
The power transformer has two secondary windings:
- 6.3V, with a center tap connected to ground for hum balancing, that feeds the heaters;
- 2 x 300V (approximately) for the anodic supply (high voltage).
The high voltage is rectified by two solid state diodes (BY127) and then goes to two main filtering cells:
- an LC that feeds the power amplifier (point A anodes, point B screen grids), these are the points at highest voltage and lowest output impedance;
- an RC followed by other two feeding the reverb circuit (points F, G and H).
At the output of the LC we have other RC cells:
- one for the vibrato circuit (point E);
- two in series for the preamplification stages (points C and D)
This is a very nice layout, every stage has its own decoupled power supply line, there is no mutual interference at power supply level. I like it very much!
And now a little note for restoration. All the electrolytic capacitors are “vintage”, mine are dated 2/67. It is suggestible to change all of them, this is true for every “over 30” electronic equipment. Old capacitors have the “nice” habit of short-circuiting and sometimes they even explode! In the occasion I will increase their values:
- the 47uF + 47uF 500V (points A and B) will be replaced by a 100uF + 100uF 500V;
- the 16uF + 16uF 400V (points C and D) will be replaced by a 50uF + 50uF 500V;
- the 16uF + 16uF 400V (points E and F) will be replaced by a 32uF + 32uF 450V;
- the 16uF + 16uF 400V (points G and H) will be replaced by a 32uF + 32uF 450V.

Channel 2

Why not channel 1? First of all because part of channel 1 layout is identical to channel 2, then cannel 2 is simpler
This stage is very simple, it has two gain stages with the tone and volume controls between them. There are two inputs:
1 high gain
2 low gain
The signal then enters the first gain stage, a standard common cathode circuit, then it goes into the passive tone control network. We have two separate controls, one for basses and mids, the other one for highs. The linearity of this network is for me a real pain in the neck: with both control knobs set at mid position the output is everything but linear. First of all the bass/mid potentiometer is not a linear one, but it is logarithmic, so at mid position, looking at the resistances from the cursor side, we have one half of roughly 150K and the other of 850K. This leads to not having a flat response at mid position. Actually the flattest overall (from signal input to power output) is achieved, in a simulation with the computer, with the bass+mid at minimum position and with the high slightly before the mid position. Moreover, moving the bass+mid control fully clockwise (maximum position) a hole of about 20dB is created around 700Hz. This means that when notes near that frequency (please don’t ask me which notes!) are played with such a setting almost nothing can be heard at all. Well this is just a simulation, I will check as soon as I will have the amplifier working, but if it will be confirmed I will rework this network to have a more linear behaviour.
The output of the tone control network is fed to the volume control then the signal goes through another amplification stage similar to the first one but the decoupling capacitor paralleled to the cathode resistor this time is 100nF and not a 25uF as in the first stage. This leads to a different amplification for basses+mids and for the highs where the gain is higher. I suppose that this is the first attempt to make the overall frequency response more linear (another attempt is found in the power amplifier stage). At the output of this stage we find a high frequency compensating network that goes to the power amplifier output. That’s all for channel 2.

Channel 1

Well this is really complicated. This channel apart for amplifying the input signal it also adds the vibrato and the reverb effects. Let’s see how it works. We have the usual high (1) and low (2) gain inputs that go to the first amplification stage. This one is a little bit different from the one employed in cannel 2. The bias of this stage is achieved through the grid current flowing in the 4.7M resistor connected between the grid and ground and decoupled from the input by the 22nF capacitor. This was a common way of biasing small signal tubes… well I’ve never been able to understand the benefits: compared to the usual common cathode layout we can only save one resistor. What a saving!!! But we gain more noise generated by the high value grid resistor. But this is how it was engineered, so let’s keep it this way. At the output of this stage we find a big mess! Well, not really. We have a passive divider that is connected to the vibrato circuit and also to the second amplification stage, then we also have a high pass filter that feeds the reverb circuit. Let’s begin with this one. The reverb has three basic components: a power amplifier (1) that drives the reverberation unit (2) whose output feeds another amplification stage (3). The power amplifier is implemented using the power pentode contained inside the ECL86, it is connected in triode mode and the decoupling cathode capacitor is of relatively small value to have a higher gain at high frequencies: this is OK, it is part of the reverb effect. The reverberation unit is a 4 spring type, it is a Gibbs/Hammond type 4 unit (still in production as I’ve read in the internet). The output of this unit feeds an amplification stage that drives a passive tone control network, then we have another amplifier to regain what has been lost in the tone control circuit. The output goes both to the reverb/vibrato pedal (for enabling/disabling this effect) and also to the intensity (volume) control.
The other effect is the vibrato. Actually it modulates a signal in accordance to a low and adjustable frequency oscillator. All the circuit drawn is the original one, but the two potentiometers that set the speed/frequency and intensity where missing in my amplifier, so searching the internet for other amplifiers employing such an effect with the same circuit I deducted their values. The speed (velocità) control value is not critical, it just sets the frequency of the oscillator, if its value is too small the frequency will be slightly higher than the original one, on the other side if its value is too high the frequency will be lower then the original one. I preferred to have a slightly high value to achieve a wider frequency range. I also put in series to the potentiometer a resistor, this sets the highest frequency for the oscillator, in this case is around 12Hz. In this circuit we have two stages: the low frequency oscillator and the neon bulb driver, all implemented using one ECC83 (V6). I quote a very nice and detailed description of the oscillator I found in internet.
Understanding Tube Vibrato Circuits
---
The vibrato oscillator is simply a phase shift oscillator. … What makes an oscillator? The answer is an amplifier with positive feedback. Negative feedback (180°), will reduce the gain of an amplifier. That is how opamps work. They are near infinite gain amplifiers and negative feedback is applied to "tame" the gain to the desired value. Another example of negative feedback is the PRESENCE control of a tube amp. It feeds back the output signal into the power amp input 180° out of phase, but only the low frequencies. This reduces the low frequency gain. Positive feedback causes oscillation. This is achieved in the circuit below by using three resistor capacitor networks. A pure capacitive network is a 90° phase shift. The series capacitor and shunt resistor network in this case provides about 60° of phase shift. Since the tube amplifier output to input is 180°, following the output with three 60° RC networks (3 x 60 = 180), results in 360° of phase shift, which is positive feedback. … The other part of the circuit is the tube that drives the opto-isolator. The opto-isolator is nothing but a neon lamp in heatshrink tubing with a light dependent resistor (LDR).

If the light is on, the resistance is low. If the light is off, the resistance is high. The LDR is shunted across the vibrato channel output, and effectively amplitude modulates the signal. The intensity control applies the LDR to the signal, or shunts it more to ground, causing less vibrato.
---
The network connected to the cathode of the neon bulb driver goes to the vibrato/reverb pedal, when it is ground connected (it is by default because of the switch installed in the front panel jack) current flows in the triode and, of course, even in the neon bulb, so the effect is engaged, when the network is not connected to ground (switch open) no current flows and the effect is disengaged (this can only be achieved using a pedal/footswitch). The LDR is connected to the intensity control whose cursor is linked to the first amplification stage output passive divider (part of the mess I was talking about before) and also to the output of the reverb effect where we have another passive divider. So the vibrato is applied to both the signal of the guitar and to the reverb. This looks correct to me. The impedance seen by the passive dividers is that of the LDR that continuously follows the oscillator, so also the attenuation of those passive dividers changes accordingly to the oscillator. Simple! The overall gain of this mess is approximately 1, no gain! So we have another amplifier, identical to the one employed in channel 2, apart from a little high frequency compensation at the input (the 100pF capacitor between the anode and the grid of the first triode) and the network at the output, probably compensating the different behaviour of previous circuits.
A little note about the vibrato intensity control. The value of this potentiometer is critical because:
- it changes the amount of the effect added to the other signals (guitar sound and reverb effect);
- it affects the overall gain of the two stages, the lower the value, the lower the gain;
- the amount of the vibrato applied, independently from the knob position, is strictly dependant from its value, the lower the value, the lower the amount but with higher values setting the amount becomes difficult;
- moving the knob affects the overall gain of the two stages, the higher the value, the higher the interference.
I made some simulations and I chose its value (50K) taking in account the best balance between interference with the gain and the amount of effect in the working range of the LDR. Probably it is not the original value but it should work fine.

Power amplifier

The signals coming from channel 1 and channel 2 preamplifiers are mixed at the input of this stage where we have a high pass filter, I suppose that is used not only to cut low frequencies, but also to compensate that funny behaviour of the tone controls. The first amplification stage is a phase inverter (I call it a differential amplifier) whose outputs feed the output push-pull stages. We have an interesting implementation: instead of paralleling tubes two by two as it is usually done, here we have two identical independent output stages each using two EL84 and one transformer, the outputs of the transformers are paralleled. The output stage is pure pentode as in most cases (Vox, Marshall, Fender, etc.) and not ultralinear as in some Selmer amps of the same years. We also have negative feedback from output to input (the differential amplifier). It is quite a bit, so I suppose that this power stage has not a “soft clipping”, it keeps on singing up to almost full power then it suddenly starts screaming. Vox AC30 and AC15 have no negative feedback. At the moment I can't tell you exactly if the output tubes are class A or class AB biased, for sure they are not class B because for this class of operation a fixed bias is needed and in this amplifier automatic polarization is used; most likely, reading the datasheet and considering the cathode resistor value, the polarization should be for class AB operation. There is another interesting feature of the output stage: all cathodes are connected to a single un-bypassed resistor. Well this can be done with class A push-pull output stages because the current flowing in that resistor is constant all over the working range of the output devices, as a consequence also the voltage drop across it is constant and therefore there is no need for a bypass capacitor… in theory, because this works only for devices with identical characteristics and in real life this does not happen even in the case of a strict selection of the devices. In class AB the current is approximately constant only with small signals (i.e. a few watts at the output), with higher signals the output tubes alternatively switch off (this is how class AB works) and the cathode current is not anymore balanced, it keeps changing and so does the voltage drop across the cathode resistor. This leads to a local negative feedback that decreases the gain of the stage and, at the same time, increases the output impedance and, of course the damping factor of the output stage (the basses usually become muddy). That’s why in class AB and class B output stages for audio use a bypass capacitor is always used, even for class A ones. If you take a look at the Vox AC30 schematic you will find the capacitor (250uF). This capacitor is employed also in the Eko Herald, Duke and probably also in the Zodiac. In my amplifier there is no clue of its previous presence, as soon as I will have the amp back working I’ll make some measurements and listening sessions and I’ll decide if it is necessary or not; by the way there is already a 470uF 63V capacitor waiting on the desk! There is a suitable place for it, it looks like that this amplifier was engineered to have it but someone decided not to install it!

So when someone says that inside an Eko Prince or Viscount there is a Vox… unfortunately he is not right, inside an Eko there is an Eko, a Vox is totally different, only the tubes are the same. As a reference here is the Vox AC30 Reverb schematic I found in internet. Quite a bit different, isn’t it?


Well, that’s all guys! I really like how this amp is engineered and how it is built. The case is rock solid (and heavy!), the chassis is really well constructed and the components are of high quality: I only found some resistors out of tolerance, most of them are within 3% (they should be within 5%) of the declared value. Only the electrolytic capacitors have to be changed because of their age.

I really wanted to post the schematic and these notes, I hope they will be useful to someone. They are for my restoration project. I already have in mind some little tweakings I will tell you about in the next weeks or months.

I’ve already disassembled the electronic circuit and started its restoration, I will be back to you with some tips to have old amps back as they were when new.. or almost!
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Tof the F*up guy
Vintage Total utilisateur


Inscrit le: 18 Nov 08
Localisation: Baltimore, MD USA

Hi Stephano,
You got some great gut's shot in this thread, it looks like an awesome project for this pure vintage amp.
Keep doing a good job.

_________________
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Edité: je corrige souvent mes erreurs, et mes fautes!
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lokrian
Vintage Total utilisateur


Inscrit le: 29 Dec 04
Localisation: 59 Nord, le Quesnoy

very good job , i will see the guts in my own !

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Stefano267
Inscrit le: 16 Oct 08
Localisation: Autre

RESTORATION HAS BEGUN!
PART 1
Well guys after the schematic (thanks for the feedback ) it’s time to work on the hardware. Meanwhile I have already bought the needed electronic components and other things:
- the missing vibrato controls knobs, fortunately the are the same ones used for Eko electric guitars so I had no problem in finding them on eBay;
- all the electrolytic capacitors;
- all the low value ceramic capacitors (56pF, 100pF and 470pF) that will be replaced by silvered mica ones;
- all the resistors that are out of tolerance, broken, missing or in bad shape;
- all the front panel jacks and even three for the outputs (2 for the loudspeakers and a stereo one for the headphones);
- all the potentiometers, although most of the original ones seem to work fine, being 40 years old it is better to change them to avoid the risk of having strange noises coming out of the loudspeakers while operating them;
- two 12” loudspeakers, now I’m searching for the box;
- tolex;
- handle.
At the moment I have no clue about leakage of the other capacitors, I can only test their capacitance value but not the leakage, this will be performed later applying the high voltage (i.e. turning the amplifier on) without the output tubes. Why without them? Simple if the two 100nF coupling capacitors are leaking the output tubes would be seriously damaged, there is not such a risk for the other tubes thanks to the high value anode resistor that limits the current flow. Meanwhile I’m looking around if I can find those “mustard” Mullard capacitors (19 of them) at a reasonable price. Nothing so far but I'm taking in consideration also paper in oil (PIO) capacitors.

Power transformer
Now it’s the power transformer’s turn. That’s how it looked before the plastic surgery.

After removing the case I had a surprise: the primary winding wires are OK…

… but not those of the secondary windings: look at the high voltage wires (two green and one red) almost completely without insulation! Also the low voltage wires (the yellow ones) are not in good condition.

No way! It can’t stay that way! I replaced and insulated those wires.


End of the job with the zinc painted shields replaced.


Reverberation unit
This is a Gibbs/Hammond type 4 spring reverberation unit, unfortunately at the moment I have no other information available about the exact model. The outside did not look nice at all.

Fortunately the inside was in very good shape with no rust on the springs.

I mechanically removed the rust with sand paper, then I painted the case with a zinc based paint, changed the RCA pins and the rubber noise insulators.




Output transformers
No big job was necessary for them, only some cleaning and black paint.
Before…


… and after




Soon I’ll be back with part 2: inductor and chassis.
Ciao!
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kleuck
Vintage Méga utilisateur

Cet utilisateur est un fabricant d'instruments et matériel audio

Inscrit le: 24 May 07
Localisation: Bas de l'Aisne

Great work, thanks for this nice addition to my schematic collection !

_________________
"Le métal, c'est plus facile assis, : c'est une musique de salon finalement !" (bonniwell, ex-métalleux)

"Oh justement, moins on en sait, plus on est capable de réellement juger quelque chose. Je suis peut-être pas expert en art, mais j'ai deux yeux, comme tout le monde, je sais distinguer un truc moche d'un truc beau comme n'importe qui d'autre.
Si Van Gogh a passé toute sa vie pauvre et incompris, c'est parce qu'il faisait de la merde, point, il ne savait pas peindre. Des années après sa mort, des "experts" ont décidés que c'était un génie, ça ne change pas pour autant son travail." (King V expert es bon goût)

Le Gecko :http://www.guitariste.com/forums/pedales-effets-homestudio,le-gecko-el[...]html
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Stefano267
Inscrit le: 16 Oct 08
Localisation: Autre

kleuck a écrit :
Great work, thanks for this nice addition to my schematic collection !

You're wellcome! Thanks for the appreciation, I really hope it will be usefull for all those having troubles with Prince and Viscount. It helped me in trouble shooting a couple of issues this amp had when I found it. Well it had more at that time, but at least it was possible to hear some sound coming from the loudspeakers until it stopped playing
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Stefano267
Inscrit le: 16 Oct 08
Localisation: Autre

PART 2
Inductor
Another fast and easy job: some cleaning, light rust removal plus transparent and insulating paint to avoid future rust. The insulation of the two wires was broken in some points, I fixed it with heat-shrinkable tube.


Chassis
The case and the chassis restorations are the biggest jobs of this project. First of all I removed all the electronics from the chassis: components board, front panel jacks, potentiometers, transformers, inductor and the two big electrolytic capacitors. Here are some pictures of the naked chassis.




The chassis was slightly bent in some places, probably because of the heat coming from the power transformer and from the output tubes or because of some hits during transports. I straightened it and then I removed the rust, first mechanically (sand paper) and then chemically with a first bath in phosphoric acid, also useful for rust proofing.


Some weldings were needed to restore cut away parts and to fix the two wands between the front and the back sides.



The chassis is now almost ready for the final work. One electrolytic capacitor needs a larger hole to fit its pins and four more holes for the clamps’ screws of both capacitors. The original sockets are in such a bad condition that all need to be replaced: all retainers are missing and the silvered contacts have no more silver attached to them, only thick oxidation and rust. Sockets identical to the original ones (noval, bakelite, chassis mount with retainers) are not available anymore, there are only the ceramic ones. I chose the type with normal contacts, because the gold plated ones have too little gold on them and this causes more problems than those the plating is supposed to solve. The new sockets need larger holes because they are bigger than the original ones and, due to the different orientation of the pins, new holes for the screws need to be drilled.

I’m taking the chassis back to the blacksmith to complete this job. After the restyling for further rust removal & proofing the chassis will spend some more hours in a phosphoric acid bath and it will be ready for the final finishing.

Originally the cassis was lightly chromed, but now all the chrome has been detached from it because of rust, bends & cuts, weldings and holes enlargements. I think that the final look would have been horrible with the chrome back again and so the cost would not be justified. I painted it with a zinc based paint exactly as I’ve done with the power transformer and with the reverberation unit.


After the painting I started reinstalling the components: irons, two big electrolytic capacitors and tube sockets with retainers.




Ciao!
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Stefano267
Inscrit le: 16 Oct 08
Localisation: Autre

PART 3
Knobs
Originally there were two knobs missing, those of the vibrato controls, and one more is almost gone while disassembling the amplifier: although I tried very hard I was not able to unscrew its screw, so I had to take it out of the potentiometer in a “hard” way. Luckily I found three knobs on eBay in a more than excellent shape! I then cleaned all of them and enlarged the holes to be able to fit them on the new potentiometers.


Jacks and Pots
The potentiometers are all Alpha apart for one CTS for the vibrato speed control. I’m going to change the mains switch too because in the original one part of the chrome has gone away.

The jacks are all Switchcraft apart from the footswitch and the phones ones. You can foresee some add-ons I’ve already planned: this amp will have phones output and two jacks for the loudspeakers, obviously plugging the phones in will disable the loudspeakers, it’s a very little and super fast add-on that will make in home use more comfortable.


Tubes
It took me three hours of searching in my OOS (Old Old Stock) and testing using my AVO Mark III mutual conductance tube tester, but finally I have all the tubes for the amp tested and matched. The result is:
- three Telefunken ECC83 (one with long plates);
- one Siemens ECC83;
- one EK ECC83, it tested much stronger than the others and will be installed as the driver for the output stage;
- one unknown brand ECL86;
- four 6P14P/EL84M Reflector military Russian tubes.
The matching I’ve done for this amplifier, considering it is not audiophile/high end, is for me very satisfactory:
- ECC83 within +-3% for each section of each tube, +-5% between tubes;
- EL84 +- 5% between tubes.

A little note about tubes. I do not like brand new (chinese) tubes, they test and sound much worse then old ones, no matter if they are NOS or used but in good condition. Nowadays NOS tubes have terrible prices, so brand new ones are the only choice for projects that are not “cost no budget”. For my audio equipment I only use military grade Telefunken, Siemens, Mullard, Philips, Amperex and Tungsol tubes, in my opinion they are the very best; in the future I’ll give a try to Russian NOS tubes, so far I only tried their equivalent to 6SN7 but I didn’t like the result, red boxed low microphonic Tungsol are “miles away”.

Circuit board
It’s time to lay my hands on the electronic circuit board! While reverse engineering this amp I spent some time in testing all the resistors to place an order for all the electronic components. Apart from the “mustard” capacitors that will be tested with power applied to the amplifier, electrolytic capacitors, small capacitors (56pF, 100pF and 470pF), potentiometers, solid state diodes, shielded cables and out of tolerance resistors are all going to be replaced. Because of the increased filter capacitors value I decided to employ bigger diodes: BY255 rated 1300V 3A, stronger that the original BY127 rated 1250V 1A.
I decided also to replace the original coupling capacitors in the critical points: the reverberation unit driver and the output tubes. The others are not critical because, in case of leakage the maximum current flowing in the following tube is limited by both the anode and cathode resistors that have high values. If the coupling capacitor of the reverberation unit driver fails, the input stage of the reverberation unit would be blown away. On the other side if one of the coupling capacitors of the output tubes (those before the control grids) fails the current in two output tubes would raise dramatically leading to sure damage of the tubes and, in worst cases, of the output transformer too. These are definitely issues I want to avoid, so here we have brand new Audyn Cap capacitors waiting to be installed on the circuit board.

Let’s take a look at the original circuit board. It’s very well engineered and constructed, the layout is neat, especially the ground paths have no loops at all, not only considering the board itself, but also the shielded cables to and from the reverberation unit, to and from the front panel jacks (inputs and footswitch), to and from the controls (volume and tones), moreover the grounding is done at the output of the circuit, as it should be. There are tons of articles about grounding schemes to be used in audio equipment (CD players, amplifiers, etc.). An in depth look at this 1967 amplifier tells you all. Great school!
Front side…

… and back side.

As you can see while disassembling the amplifier I put some stickers on all the cables I detached from the transformers and from the tube sockets: this will be of great help when I’ll put everything back together because I will not have to continuously check the schematic diagram.
Here all the replacements.


After some time with the soldering iron in (and on!) my hands here is the result.


Ciao!
Haut
lokrian
Vintage Total utilisateur


Inscrit le: 29 Dec 04
Localisation: 59 Nord, le Quesnoy

telefunken diamond are bestin this amp

_________________
.:: LA BRADERIE DE LOKRIAN !! ::.
http://www.guitariste.com/forums/viewtopic.php?p=3653778#3653778

CHERCHE ZIKOS dans le 59 pour jouer du bon gros Rock"n"Roll / hard /glam et même métal ! http://www.guitariste.com/forums/petites-annonces-musiciens,59-guitari[...]1090
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Stefano267
Inscrit le: 16 Oct 08
Localisation: Autre

lokrian a écrit :
telefunken diamond are bestin this amp


Hi lokrian,
thanks for your suggestion. In my extremely messy lab I still have some places where to look for other ECC83s. Last time I've been at my father's place I didn't have much time to both look everywhere and to test&match all tubes. When I read your reply I recalled I have some other tubes I didn't find while searching for all replacements. Somewhere I have some 50 russian 6SN7 (6H8C) I wrote about that I did not see, this means that I did not search in every place. Awful mess !
I know for sure that somewhere there are also the original Telefunken EL84 that I used in a vacuum tube integrated amp I made some 10 years ago. I know they are there, I never throw away tubes!

During the Christmas holidays we took our nephew to a big musical instruments store where he had the opportunity to test... well let's say play with a lot of acoustic and electric guitars. He really liked a Gibson, I don't remember the model, only its golden color and its tag price... 2500€ After playing with a couple of Marshall, one Fender and one Pevey amps I connected the guitar to a Vox AC30. Astonishing! We all really loved that sound. The difference in sound dynamics and richness was dramatic... of course only in my/our opinion.

Lokrian, by the way, can you please tell us a little bit of this amp? How does it sound like?

Have a nice day!
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