Voice of Music Amp Recomendations for a Friend

[Platefire]

This is a internet friend that lives way away from me but he has sent me several amps to rework for him. He also likes to attempt to convert amps himself. This is one. He wanted me to suggest what to do to make the breakup more pleasing. I told him send me a schematic and I would take a look. So here is his comment and schematic:

Attached is the VM schematic. Appreciate your taking a look for anything that might be causing the kinda bad overdrive when cranked up. Sounds good clean and just barely breaking up, but louder than that not so good.

My first thought was tear out all the strange stuff and make a champ out of it but the tone stack is really different than I've seen. Looks like its has NFB from speaker tap. Kind of different. He does like the clean sound. So before I say anything to him, thought I would request some of the boards opinions. Thanks in advance. Platefire

[VMS]

Has he tried different speakers?

Also the nfb-loop might be the culprit or lack of grid stoppers.   

[sluckey]

make a champ out of it

agree

[VMS]

That is interesting circuit, fixed-bias and cathode connected to the secondary side of the OT.

So if I'm not mistaken it has two negative feedback loops??
 

[Platefire]

Yeah strange! I was thinking, maybe if you disconnected the NFB the breakup might improve but might disable the crazy tone stack. Volume pot before the first gain stage, no cathode bypass caps, 220k & 150K plate resisters(wonder what the plate voltage is?)

sluckey, where the 6V6 cathode goes to the OT, if you made a champ out of it would you just disconnect from the OT and use a standard champ cathode resistor and bypass cap to ground? Platefire

[HotBluePlates]

That is interesting circuit, fixed-bias and cathode connected to the secondary side of the OT.

So if I'm not mistaken it has two negative feedback loops??

2 negative feedback loops, one of which is the 32Ω secondary tap. But not "fixed bias" but "back-biased".

All of the main filter caps are connected to the 0v ground of the rest of the circuit. But the high voltage winding CT is not grounded, it is connected on the far side of the  300Ω resistor also connected to the filter cap grounds. The current of the entire amp flows through that resistor, creating a voltage drop which makes the CT appear "negative relative to ground" ("ground" being the filter cap - terminals). A dropping/isolation resistor and a bias cap complete the back-bias supply.

This schematic is also an example of using series resistors between the PT winding & rectifier plates to keep current within the rectifier's rating, in spite of the large 1st filter cap (and an example of why I always say data sheets never have a "max filter cap" value).

Yeah strange! I was thinking, maybe if you disconnected the NFB the breakup might improve but might disable the crazy tone stack. ...

There are 2 pick-off points for feedback: the 32Ω tap to the 6V6 cathode, and the 8Ω tap for the tone controls. I'd first try unhooking the 32Ω tap, connecting the 6V6 cathode to ground and seeing how it sounds.

... Volume pot before the first gain stage ...

They appeared to expect a pretty hot input signal from the mono input. There is a voltage divider which knocks signal down by 2/3 from that input, before it ever gets to the Volume control. The Volume control will knock it down further, which is not good for signal-to-noise unless the input signal is quite hot to begin with.

The problem is the clever tone controls are enabled by the feedback loop from the 8Ω tap, and that loop is returned to the 1st 12AX7 stage's cathode. You can't move the Volume control to the typical place after that stage, because then it's inside the loop. It would probably react oddly with the feedback & tone controls when adjusted.

If you friend agrees to lose the tone controls, then the other suggestions about Champ-like wiring are available for use. Do leave the 6V6 cathode connected to ground, keep the back-biasing & resistors from PT to rectifier, use a typical Fender input circuit (even if just a single jack) and move the Volume control to its typical position between 12AX7 stages. After that, he can evaluate how it sounds & whether either of the cathode bypass caps are needed.

A Treble control (sans feedback) can be implemented using a 0.1-0.5µF cap as a cathode bypass, but with a pot wired as a rheostat between the cap & ground. Turn up for less resistance, and gain for trebles is boosted. When turned down for more resistance, the cap is decoupled from ground, allowing local negative feedback to reduce that treble-gain.

A Bass control can be implemented by placing an extra coupling cap (maybe 0.01-0.022µF) in series with the existing 0.047µF coupling cap between 12AX7 sections. Place a pot wired as a rheostat across the new cap. When the pot is turned up for less resistance, the new coupling cap is taken out of circuit for stock bass. When turned down, the series cap is brought into play, reducing the total effective capacitance to a value less than the added, smaller cap. Bass is trimmed.

[kagliostro]

I tried to make more readable the schematic, this is what I got


First level of adjustment


Second level of adjustment


Franco

[eleventeen]

I would just turn it into a Champ, like others have said. Tweed or blackface, you decide. Other than a cap or resistor here or there and the bass/treble control pots, everything is there. I would imagine the existing power supply is well below the +360 vdc that a blackface Champ wants to see but might be OK @ 305 volts like a Tweedy wants. A tweed circuit only uses a single 1M pot and you already have that so it would be cheaper. Remove (or reduce the values of)  those 500 ohm resistors in series with the 6X4 plates and B+ will rise. E-cap replacement probably called for; both for the fact that they are old, but if you go blackface they could be over their spec. Although I tend not to like 6X4s and 6X5s, there are millions of record players and even test gear that use those tubes and they are cheap. Free, in this case! The 6X4 is well within specs either way.

[drgonzonm]

a quick perusal of the internet indicates this is a hifi amp.  Someone get an exorcist, we can't repair hifi;s on this forum

[HotBluePlates]

... I would imagine the existing power supply is well below the +360 vdc that a blackface Champ ... Remove (or reduce the values of)  those 500 ohm resistors in series with the 6X4 plates and B+ will rise. E-cap replacement probably called for ...

The Champ (at least the later ones with the 125P1B PT for which we can easily know specs) used a 325-0-325v winding. This PT is 350-0-350. The original circuit had the 6X4 and a 40µF main cap. While e-caps will likely need replacement, we can be confident will will have too much voltage and too-big a 1st cap to cut out the 500Ω resistors.

We should find out what voltage is appearing on the filter caps for this amp before changing any aspects of the PT/power supply/bias method. They will be sure to work for a 6V6 driven by a 12AX7, and I'm sure the owner doesn't need every milliwatt of power output if the supply is lower than a tweed Champ's.

I only point this out as one of those places "When Good Conversions Go Bad" since everything about the PT, power supply & output stage can be kept while only modifying the preamp and feedback to arrive at a useable guitar amp.

... we can't repair hifi;s on this forum

Speak for yourself! 

Besides, this is being turned into a guitar amp.

[Paul1453]

Guys like HBP can understand this circuit, but for KISS (Keep It Simple Stupid) techs like me, it's a NO-GO.   

While the back bias/neg feedback/CT connection etc might have worked well for what it was.

It is too complex for someone like me to fix tone issues on.

I could troubleshoot and repair this circuit if it had a faulty component.

But if it is working and just not sounding good, you really have to understand how all these things work together to improve it's tone.

I'd be looking to rewire what is there, more like a Champ.   

[Platefire]

Ok, thanks for all your good input. If I am interpreting things right the consensus is that there in no easy or good way to make the existing circuit sound good where it would break up nice when cranked in it's original configuration or just by making some very minor changes.

I think he was looking for an easy way without a complete rewire to still get his existing clean sound but get an nice harmonically balanced OD as he progressively cranks it up.

So I'm going to tell him to get what he is looking for, it will take a complete rewire of the circuit and if he is interested in that the first thing I would need to advise him is the existing voltages readings on that 40uF, 20uF and 10uF filter caps to see what kind of champ circuit it might be rewired to. Does that sound OK? Platefire

[eleventeen]

I think what folks are suggesting (and I apologize, I did not see the 350-0-350 rating on the PT, as HBP said keep those R's in series with the 6X4 plates) falls quite a bit short of a "complete rewire". The heaters are done, including the humbucking pot. The output section is done, the power supply, maybe you have to replace the ecaps but other than that it's done except for moving one wire. I think one could buy about $15 in parts and freight, change about 25-35 solder connections and have a BF Champ. One at a time, mark them off the schematic with a colored pencil, usual story. I don't consider that all that ambitious, but I suppose everyone has their individual limits on what they can imagine themselves doing.

[Platefire]

Yeah thanks, I understood that the power supply and and output section pretty much needs to be as is same maybe preamp plate resistors or replacement filter caps. If I was doing it myself would remove all the preamp wiring/tone stack up to the power tube grid and rebuild champ from input to power tube. I'm not to familiar with a blackface champ but do they have bass and treble tone controls but before I go to far on planning what the rebuild will be, need to let him know that from input to power tube needs a tear-down and rework. If I know him, he will not want to take that on and either leave it as is or send it to me for a rework, we'll see. Thanks for the help figuring this out. Platefire 

[kagliostro]

Just a thing to give a try

replace the plate resistor of V1b with a 120K pot + 27K resistor (in series)

and take the signal to feed the power tube from the wiper of the pot

if a good compromise setting is found replace the pot with fixed resistors

(may be you can do the same with V1a - using appropiate resistors & pot values)

Franco

[Platefire]

I sent my friend an e-mail giving him the news that it would take a preamp/tone stack rewire to get where he wanted to go. I haven't got any reply so far. Platefire