Stromberg Carlson SAU-10 Convert to Guitar Amp.

[Pinball]

I’m trying to convert a Stromberg Carlson SAU-10 to a guitar amp. I have already converted the mic input to a 1/4 jack. I added a 68k grid stopper going into the grid of the 6av6.
I want to convert the tone to a Fender style tone. I also thought about using the first volume as a “gain” between the 6av6 and 6au6. From the plate of the 6au6 to the tone pot-“master volume” pot - grid of 6L6.

My questions are :

How should I wire the volumes and tone.

Should I increase the plate voltages of the 6av6 and 6au6.
Should I rebuilds those tubes bias to be more guitar friendly.

Should I change any of the B+ caps or resistors?

Any other advice?

Schematic is attached.

BTW I did get the amp working prior to starting the conversion.

[roarshock]

That amp looks fun. I'd recommend looking at a Fender Princeton 5F2a schematic and using that tone control. Really, you'll need to know what you want to get the best advice here.  I'd consider reworking the bias on the 6AV6 to be cathode biased instead of gridleak biased, but that's just my opinion. I do have an amp that I love that is grid leak in v1, so grains of salt might need to be taken. With how unsure you are of what to do, I'd recommend doing as little as possible while keeping the amp safe and operational. From that perspective, how old are those caps in the power section?

[mountainhick]

 Few thoughts of what you can do, but just choices;

If the electrolytic caps are original, yes, change them all they are OLD.

I would take out the current phono input and its tone control.

Yes to 5F2A for tone control.

The B+ is in lower range for 6L6GC guitar amp. You can goose the voltage using another rectifier tube with less voltage drop. I would be sure to keep that within the 5V 2A rating for 5Y3. Many rectifier tubes draw more current, so a couple choices: GZ34, 5Z4, 5V4. Run a load line to confirm you're still in good territory.

I am unsure with the grid biased input stage, but based on 12AX7 spec sheet info, I think you can get more out of that 6AV6 triode (equivalent to a single 12AX7 triode). I personally would roll with that before switching to cathode bias to find out how I like it.

Note: be sure C1 is still good, not leaking. It keeps the grid bias voltage off your guitar strings.

The bottom of the chart with zero cathode resistance Rk=0, are grid biased options. The 3.3meg in your schematic provides less bias than the 10meg in that chart.

Also notice in those configurations, there are Rs variations from .24 to 1M. You can replace the 500K voume pot with 1M for more gain.

[Pinball]

That amp looks fun. I'd recommend looking at a Fender Princeton 5F2a schematic and using that tone control. Really, you'll need to know what you want to get the best advice here.  I'd consider reworking the bias on the 6AV6 to be cathode biased instead of gridleak biased, but that's just my opinion. I do have an amp that I love that is grid leak in v1, so grains of salt might need to be taken. With how unsure you are of what to do, I'd recommend doing as little as possible while keeping the amp safe and operational. From that perspective, how old are those caps in the power section?

Yeah I already changed the first stage to cathode biased.

The electroytics are fine. I checked with an ESR and brought them up very slowly over a 1 day period.

I changed the coupling caps to .022uf.

My main questions are:
1.  That 6au6 and how to treat the screen. Should values be changed for a second stage of a guitar amp.
2. Is creating a "gain" pot between v1 and v2 plus a classic Fender volume tone between v2 and v3 a good idea?
3. 6av6 and 6au6 plate voltages are low and I know this amp is voiced more for a PA and not a guitar amp. What should I change.
4. Is C8a really needed. Is R19 needed.

Thanks,
Tim

[SEL49]

4. Is C8a really needed. Is R19 needed.

C8a is absolutely needed. R19 is not really needed but it's a good idea because it will help drain the filter caps when the amp is turned off. I would let it be.

[mountainhick]

That amp looks fun. I'd recommend looking at a Fender Princeton 5F2a schematic and using that tone control. Really, you'll need to know what you want to get the best advice here.  I'd consider reworking the bias on the 6AV6 to be cathode biased instead of gridleak biased, but that's just my opinion. I do have an amp that I love that is grid leak in v1, so grains of salt might need to be taken. With how unsure you are of what to do, I'd recommend doing as little as possible while keeping the amp safe and operational. From that perspective, how old are those caps in the power section?

Yeah I already changed the first stage to cathode biased.

The electroytics are fine. I checked with an ESR and brought them up very slowly over a 1 day period.

I changed the coupling caps to .022uf.

My main questions are:
1.  That 6au6 and how to treat the screen. Should values be changed for a second stage of a guitar amp.
2. Is creating a "gain" pot between v1 and v2 plus a classic Fender volume tone between v2 and v3 a good idea?
3. 6av6 and 6au6 plate voltages are low and I know this amp is voiced more for a PA and not a guitar amp. What should I change.
4. Is C8a really needed. Is R19 needed.

Thanks,
Tim

ESR is not the be all end all to filter cap testing. Cheap ESR testers use low voltage. Caps can still leak at B+ voltages.120Hz hum would tip you off.

I would keep the vol/tone between V1 and 2. YMMV. I suppose you could do a TMB after the 6AU6.

At 240v, the B+ to the 6AU6 and 6AV6 is not that low. The low plate voltages are due to biasing and the voltage drop of the tube configurations. You can raise that B+ by replacing the 33K dropper with a lower value. You can calculate voltage drop based on current here: https://www.ampbooks.com/mobile/amplifier-calculators/RC-ripple-filter/calculator/

[jjasilli]

It looks to me that this amp may already be a guitar amp.  Personally, I would entirely rebuild the amp to spec (maybe ignore the phono input circuitry).  Then see what it sounds like.  Old out-of-spec components will not reveal how the amp should sound.  You might like it for guitar.


Often, to me, PA amps can sound dark for guitar; but that tends to assume a baxandall eq circuit (my personal quirk).  This amp has a treble bleed like a Tweed Fender, and might sound good. 


Also, the circuitry might not have enough gain for overdrive??? I would alter the amp only after these unknowns are sorted-out. 

[Pinball]

I'm just trying to find a way to utilize the three pots on the front of the amp. I removed the phono input but have an additional 500k pot just hanging out with nothing to do.

[bmccowan]

I converted a Bogen CH8 which is very similar to this amp. V1 is the same, V2 is a 6SJ7 which is an octal pentode with specs similar to the 6au6 in your amp.
So since its a triode driving a pentode I looked to the Matchless Clubman for the preamp section. I had built a clubman prior and really like it. The clubman sets up V1 for pretty low gain, as the pentode in V2 develops a lot of gain.
As others here suggest, I wired V1 for cathode bias, and used a 5U4 rectifier for a bit more voltage. I like the amp a lot.

[Pinball]

Update.

Amp is up and running. Decided to just leave one pot unused. I did change the first stage to a cathode biased stage and used a mini switch to engage or disengage the bypass cap. I rebiased the second stage to be more like a fender. I added a Fender style volume/tone. I changed the negative feedback resistor to a lower value. I didn't really change a whole lot else. Amp sounds good to my ears. Thanks for tips and help!!

[roarshock]

It sounds like you're done, but an unused pot might need a purpose. You might want a master volume, a bass and treble tone circuit of your choosing, or my favorite, reverb. Happy music making whatever you do!

[DummyLoad]

Where I'd start. Please see attached.

--Pete