Need help with decisions for PA to Guitar amp convo

[K O'Connor]

[quote pid="1708" dateline="1746460309"]
You mention doing a test with an open load. Is this for the PT and OT tests specifically? Also, Aren't we supposed to NEVER run a tube amp without a load? What is running an open load test going to tell me, briefly?

There are posts about how to do power tests in the Test and Measurement section of this forum,

Thank you for this, I shall see.

Reply:
The open-load test demonstrates that the circuitry is properly connected and that the output is symmetrical up to clipping. A damaged OT may pass this test, but more likely not. adding the load and sweeping the level quickly up to clip then back down is the acid test for the OT.

The power amp is similar to many tweed-era PAs and will have a warm basic tone.

Which Tweed model or models is my little amp most similar to, or which specifically did you have in mind when you made this statement, because it would be nice to download a fender schematic that I could sit next to mine to be able to compare and to use as a reference for mod and inspiration and to better help me see the differences between the PA and the guitar of the circuit.

Reply:
Be inspired by the circuit you actually have.

There are two basic PA schemes used in MI, this one you have is the Tweed vesrion with separate gain and splitting functions (splitting via concertina splitter), the second uses the Schmitt splitter front-end. The tweed-era circuit is more toneful and warmer.

The plate filter cap is woefully undersized and should be 100uF minimum.

Which cap type should I use for this replacement. Do you have any suggestions about which type of composition should I use for the caps and resistors you mentioned for the mods you proposed?

Reply:
Stay away from voodoo brands unless you enjoy spending $$$ and just stay with common capacitor brands except for Illinois Capacitor -their plastic caps are okay but their electrolytics are very poor.

The preamp has three triodes available and this is enough gain to create any useful amount of distortion you may desire. However, this is the "Scott" amp and you are looking for your own tone. You need to define that goal and that will determine how the preamp is reconfigured. As it is now, there is only one gain stage between the input and the PA, and the PA has a low closed-loop gain.

Well, I have the most unfortunate and unoriginal obsession with the clarity, articulation and singing dynamic gain that is heard in Hendrix's live recordings, the Winterland Concerts specifically. Its just sublime and I can't help myself. And I also play single coils almost exclusively. So maybe that helps.

Reply:
Those amps would be plexi-style circuits voiced to be bright - almost unusably bright when played below clipping. You can achieve that sound with any amp.


OK. bypass themBut suppose if I were to want to just keep it simple and not use the trans sockets at all, how would I go about wiring their elimination? As it stands, I would have to  to move the inputs and bypass them obviously, right? So I should lift the wires that go from the input jacks to the trans sockets and move them to V1 and V2, respectively, with observation of proper impedence matching, correct?

Reply:
There are no impedance matching issues here. Use a 1meg grid-leak for the input stage and any guitar pickup will be happy.

If you plan to add tubes, you should add an auxiliary PT to support the extra tube heaters. You cannot count on the stock PT to support any more heater load than the stock tube set represents. If you ad this PT, you can bump its rating to accommodate the use of different power tubes, as well. Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

Hmm, makes sense. What voltage and current PT should I look for? Center tap? Any ideas for model suggestions? Of course you may not be able to answer this besause we arent sure which tubes, right? When you say octal tubes you mean like the 6QN7 (i think that is the number) type or similar?  Or do you have any expert saggestions? I think I actually have some NOS GE's somewhere. Wow this is interesting.

Reply:
The guidance for this is in the quoted statement.

Expanding on it simply means you have to look up the heater current requirement for the tubes that are in the amp now. Add that up and this is the current you can safely expect to pull from the stock PT.

Now look up the heater current for prospective tubes you might wish to use. See how much higher some of these are. TUT3 explains this process and has a list of common tubes used in MI. In general, the octal-based preamp tubes draw more heater current than the subminiatures do. Octal power tubes that are pin-compatible with what you have draw from 450mA (6V6) up to 1,800mA (KT-88).

The auxiliary PT does not need a CT. A faux-CT using resistors works better and in any case, you should use a DC-stand-off for the heaters to reduce noise, as TUT3 shows in every project.

You can simplify things by deciding to power the power tubes from the aux PT, say, in which case a 4A unit would handle any tube pair you ever want to try. This leaves ample capacity in the stock PT to support a lot more preamp tubes or even the small power tube for a reverb driver, below.

Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

This is because they will be part of the preamp and not the power amp section right?

Reply:
No, this is with respect to power tube swaps.


Could one of those trans sockets work as a gain stage for reverb? I have a cool old tank reverb.

Reply:
You can use a power tube as a reverb driver just as Fender did early on to save $$$. Their first reverbs used a 6V6 and the same OT from the Champ. Later units used a 12AT7 and a custom OT. A reverb tank only requires 150mW or less.

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