Guitar Tube Amp Conversion Questions

[ToobNube]

Hello, im relatively new to the world of tube amps, but I'm determined to learn. I've built and modded guitar pedals, and have worked with other electronics, but this is my first time working on a tube amp. I decided I'd get more out of converting a tube amp into a guitar amp than simply building a kit. I dont just want to know what to do, i want to understand why.

I picked up a Voice Of Music tube-amp-powered speaker the other day to convert into a guitar amp, and I've been thinking about how I want to do it. I'd like to keep as many of the unique qualities of this amp as possible while prioritizing a good tone. I dont want to completely gut and rewire the chassis into a common circuit, I'd like to use as much of the original circuitry as possible and make something new and unique. I suppose if i was leaning towards any of the classic tube amp sounds it would be the fender "tweed" sound, but I'm much more interested in preserving the sound this amp already has than building a tweed-style amp in this chassis. However, if I'm faced with multiple otherwise equal options and one takes me closer to a tweed sound, thats the path I want to take.

The guy i got it from said he got it from a hoarder who claimed it works. I havent plugged it in yet. It probably hasnt been turned on in decades. I think this amp/speaker combo was originally used for a turntable, so it probably needs a little work to sound good as a guitar amp.

I'd like to bounce some ideas off the community, and i also have some questions. I'm attaching a pic of the schematic as found on the inside of the cabinet, and pics of the chassis exterior and interior.

It has a 6au6 in v1 feeding into a 12ax7 before hitting 2 el84s. Im thinking about swapping out the 6au6 for a second 12ax7 in V1, adding a volume knob in between the 2 gain stages in V1, and leaving the V2 and beyond the same. Is that reasonable? If i do that, will I need to change the voltage going to V1?

It looks like the signal splits before going into the 12ax7 in V2. I was thinking about keeping it that way. Is that ok? I typically would expect to see the signal go through twice i stead of splitting and going through once each. I would think that if im getting enough gain from a 12ax7 in V1 this would be fine. Am i thinking about this correctly?

It looks like there's a "contour knob" hooked up to the second (edit: first and only) gain stage in the 6au6 in V1. I was thinking id have to remove it entirely after swapping out for a 12ax7. Any thoughts? What is a contour knob and would/could it enhance this amp?

Before i plug this in and turn it on, I want to test the transformers. I have a standard $50 digital multimeter. Is it safe for me to test the transformers? How do i do it? What should i be looking for? What else should i test or do before plugging it in?

This question is going to sound dumb, and i apologize. How can i tell how many watts this amp produces by looking at the schematic? Or any tube amp schematic for that matter? Is there a simple way to determine amp wattage?

I want to eventually replace the woofer/tweeter combo with a guitar amp speaker. How can i determine the ohm of speaker to use? Do i need to test the output transformer?

The pots for tone and volume look like they are before v1 in a little separate module. I dont fully understand how it works. Any red flags for a guitar amp? I was thinking about leaving it all as-is (minus contour knob), and then adding vol/tone later in the circuit as is normal for tube amps. I dont mind having 2 volume knobs.

Most guitar tube amps have a bunch of big capacitors, and this one only has a few, and they arent terribly large. Is that ok? Do i need to add capacitors to this amp? In theory, where would i add them and what value (edit: I mean benefit, not the capacitance) would they add?

This is an integrated tube amp, correct? Would you asses this amp to be a good candidate for a guitar tube amp conversion? Why or why not? Is there any compelling reason not to convert this into a guitar amp?

Do you have any general advice for people new to working with amps, or specific advice for this project based on the schematic?

thanks in advance

Edit: there are not 2 gain stages in a 6au6, and I incorrectly said the contour control was hooked up to the "second" gain stage
Edit: added clarity to what I meant when I used the word "value" in regards to adding caps to the circuit. I realized that word choice may be confusing.

[Enzo]

Amp wattage? Look at the power tube or tubes. What type are they? That will give you an upper limit on power. Beyond that it is details. A pair of 6L6 will be potentially good for about 50 watts. But there are amps with that pair only rated for 40 watts. Which leads me to this: power isn't loudness. The relationship there is exponential. All things equal, 20 watts is only 3db louder than 10 watts 100 watts only 3db louder than 50 watts. Don't make watts you primary focus. A tiny Fender Champ will still make plenty of loud.

[Jazz P Bass]

https://forum.jbonamassa.com/viewtopic.php?id=22362

Post 14 indicates changes for a guitar amp. (though no specifics)

Food for thought.

[eschertron]

Hello, im relatively new to the world of tube amps, but I'm determined to learn. I've built and modded guitar pedals, and have worked with other electronics, but this is my first time working on a tube amp. I decided I'd get more out of converting a tube amp into a guitar amp than simply building a kit. I dont just want to know what to do, i want to understand why.

Excellent attitude

I picked up a Voice Of Music tube-amp-powered speaker the other day to convert into a guitar amp, and I've been thinking about how I want to do it. I'd like to keep as many of the unique qualities of this amp as possible [snip]. I dont want to completely gut and rewire the chassis into a common circuit, I'd like to use as much of the original circuitry as possible [snip]. I suppose if i was leaning towards any of the classic tube amp sounds it would be the fender "tweed" sound, but I'm much more interested in preserving the sound this amp already has than building a tweed-style amp in this chassis.

Start with restoring the amp. Compare it to a vintage Vox AC15 and you'll see why.

It has a 6au6 in v1 feeding into a 12ax7 before hitting 2 el84s
It looks like the signal splits before going into the 12ax7 in V2. I was thinking about keeping it that way. Is that ok?

the 6au6 is a pentode tube (like the AC15), with much more gain than a 12ax7 stage. Replacing that is possible, but then you'll never hear how the amp was designed to sound. Also the 'split' triode tube is the Phase Inverter stage, and is used to get (nominally) equal-but-opposite signals to the power tubes. The 2xEL84 complement is also present in the AC15. I'm drooling over your find already.

It looks like there's a "contour knob" hooked up to the second gain stage in the 6au6 in V1. I was thinking id have to remove it entirely after swapping out for a 12ax7. Any thoughts? What is a contour knob and would/could it enhance this amp?

from a brief look, the contour is a variable NFB control, using a selection of resistors to enhance (or de-hance!) the hi-fi quality of the amp.

I want to test the transformers

You will want to test the transformers before sinking a lot of cash into the project, but you can wait until you've learned a lot more about amps and troubleshooting before you tackle that.

This question is going to sound dumb, and i apologize. How can i tell how many watts this amp produces by looking at the schematic? Or any tube amp schematic for that matter? Is there a simple way to determine amp wattage?

Not a dumb question at all. The simple answer to "how much" is "plenty" unless you're in a death metal/grunge band, in which case you'll want a different amp to start with. The more complex answer is that it requires some engineering chops and some actual voltage readings off the power supply. The middle answer is that 2xEL84s produce about 15 Watts on average (AC15, gettit?).

I want to eventually replace the woofer/tweeter combo with a guitar amp speaker. How can i determine the ohm of speaker to use? Do i need to test the output transformer?

A very good question, and by testing the output transformer you will be able to calculate the Ohmage needed. Some learning is involved. And no good guitar amp ever has the speakers like what comes with this amp. The speaker choice is probably the single biggest difference between 'hi-fi' and 'instrument' amps (and your choice of what guitar speaker to use will also control - to an extent - the sound you get).

Do you have any general advice for people new to working with amps, or specific advice for this project based on the schematic?

I skipped to the end... If you are serious about learning high-voltage electronics, be sure to learn about safety first. Then read all you can on this forum (& everywhere else) about the what others have done.

thanks in advance

Thank you for taking an interest.

[ToobNube]

Hey eschertron, thanks for the reply. Your comments have already helped guide me and my thinking.

I'm going to take your advice and keep the 6au6 in for the preamp; you're absolutely right that I would lose important qualities of the amp if I switched out the preamp for a 12ax7, and I don't want that. I've looked at the AC15 schematic as well as looking up other common pre-amp circuits that use pentode tubes and comment #14 here, and i believe I will need to either move or add a volume pot after the 6au6 to ensure I'm getting enough clipping, since hi-fi amps are designed specifically with clean headroom in mind. I'm looking at the schematic and it looks like the output has 25VDC which splits off before the phase inverting stage. I'm thinking I should add the volume pot after the 25VDC splits off and directly before phase inverting (to be specific, directly before the signal hits pin 2 on V2 and splits into the first gain stage and R5). I'm essentially attempting to place the volume pot in the same position as it is in this circuit here.

I believe this will serve to push the 12ax7 phase inverters, which is where the clipping will happen. Am I thinking about this correctly?

Also, am I correct that there won't be any DC voltage at that point?

I should place a cap before the pot, yeah?

If clipping happens in V2, will R5 make the clipping asymetrical?

Side note: how do amp makers/designers determine values of resistors and caps? I feel like there probably are rules of thumb and calculators to help. Could you point me in the right direction?

thanks

[ToobNube]

https://forum.jbonamassa.com/viewtopic.php?id=22362

Post 14 indicates changes for a guitar amp. (though no specifics)

Food for thought.

Wonderful, this is practically a playbook for me. Thank you.

[ToobNube]

Amp wattage? Look at the power tube or tubes. What type are they? That will give you an upper limit on power. Beyond that it is details. A pair of 6L6 will be potentially good for about 50 watts. But there are amps with that pair only rated for 40 watts. Which leads me to this: power isn't loudness. The relationship there is exponential. All things equal, 20 watts is only 3db louder than 10 watts 100 watts only 3db louder than 50 watts. Don't make watts you primary focus. A tiny Fender Champ will still make plenty of loud.

Hey Enzo, thanks for the reply. Really the question is "by only looking at schematics, how much information is a person able to derive about the wattage of a tube amp?" eschertron's response in my other post (sorry again, everyone, for splitting this into two places) was that "true wattage" is very difficult to ascertain based on only the schematics without crunching a bunch of numbers, but that there are some common wattages which are often telegraphed by the tubes used, which is essentially what you said as well. In this case, it would seem that 2x EL84s indicate around 15 watts.

I'm not focused on wattage, but I am a curious person. When you look at power tube(s) in a circuit, are there any general guidelines that help you estimate amp wattage? Eg. multiply # of EL84s by 8, etc.

[ToobNube]

Based on the schematic and the presumed intended purpose of this amp, does it appear that the amp expects the Audio Input will be Line Level, and not Instrument Level? And therefore I will need to adjust the input section accordingly? Or does it look like it's expecting Instrument Level Audio Input?

[Helmholtz]

A good resource is always the tube datasheet, which typically shows design examples.
For a cathode biased PP amp with a B+ of 300V (at full power) the datasheet specifies an output of 17W.
With 4x EL84 twice the power can be achieved.

Output power mainly depends on plate to cathode voltage and OT primary impedance but is restricted by tube limits.

[loudthud]

Did anyone notice that the plate Voltage of the 6AU6 is 25V and it is directly coupled to the phase inverter ? What's the deal with the three pots on the left side of the schematic ? Looks like a Volume control with loudness compensation and a Baxandall treble and bass controls. Is that included or some kind of option ?

[ToobNube]

Did anyone notice that the plate Voltage of the 6AU6 is 25V and it is directly coupled to the phase inverter ?

Hey loudthud, thanks for the reply. I did notice that, but it didn't strike me as unusual until you mentioned it. Looking at the circuit analysis of the AC15, i see that 90VDC is being applied to the pentode plate in the preamp section. My current theory is that since this was designed as a Hi-Fi amp, the lower voltage will create less distortion/clipping in the preamp stage. Does that track? To the end of converting this Hi-Fi amp into a guitar amp, should I experiment with increasing the 6au6 voltage?

What's the deal with the three pots on the left side of the schematic ? Looks like a Volume control with loudness compensation and a Baxandall treble and bass controls. Is that included or some kind of option ?

The audio input first goes through the volume, treble, bass controls before being connected to the amp via RCA input. There's also a "contour knob", but that is applied later in the circuit. You can *kinda* see the "Control Panel" (for lack of a better term) in one of the pictures here. I did have to remove some nuts and dismount it in order to pull everything out from the cabinet, so it appears to be a permanent feature on the amp. The input connector into the "Control Panel" has been cut, so it's just bare wires.

That does make me think, though. If RCA jacks connect the "control panel" to the amp, it isn't a stretch to imagine RCA jacks bring the signal to the amp in the first place (it was, after all, an extension speaker ostensibly for home audio). It then follows that the signal was likely being delivered at "Consumer Line Level", which is roughly -10dB. Instrument Level for a guitar is roughly -30dB, so I will need to account for that. Any thoughts?

[Helmholtz]

Even more surprizing that the V1 screen seems to be at 125V.

Changing the V1 plate voltage would offset the PI bias voltage because of the direct coupling.

Also V1 should not be considered the preamp. Rather it's within the global NFB loop and thus can be considered part of the power stage.

Trying to change gain within the NFB loop with not have much effect and is likely to cause instability.

I would disconnect the NFB. This will increase gain and would allow for more mods.

[ToobNube]

Also V1 should not be considered the preamp. Rather it's within the global NFB loop and thus can be considered part of the power stage.

Thank you for the insight. What role would you say V1 is filling? Does this amp need an actual preamp? Is it possible the amp would have expected signal to flow through the pre-amp of a turntable before getting to the powered speaker? Does this help explain the vol/treb/bas controls entirely before the amp? I could drill a hole and add a socket for a new tube, create a normal guitar preamp circuit, then feed the signal into the volume/bass/trebel "control panel", and then directly into V1 and beyond. Thoughts?

[ToobNube]

It looks like capacitor C5 has 3 prongs. I tested the other capacitors and I'm going to replace them, but im not sure what to look for to replace C5. It has 3 shapes on it with different values for each shape: semi-circle - 40mfd 250vdc
triangle - 10mfd 25vdc
square - 40mfd 450vdc

[35L6]

I would try it like it is to see what it sounds like and listen to those speakers before you ditch them , you might get lucky . Stay safe , go slow , measure everything and document everything .You need to install a 3 wire power cord .