Tube Amp Archive

How about if a different, known good GZ34 is fitted?

The original pi circuit used bootstrap biasing. The Paul C mod simply converts to fixed biasing. I don't know how the sound is affected. Your mod looks to be done correctly. The Paul C mod in the pic on the right uses the full 56k cathode resistor for autobias. It's not fixed bias as only the grid ...

When I investigated the 100V winding on a Partridge OT, it seemed to be optimised / intended for a 200 ohm load
https://ampgarage.com/forum/viewtopic.p ... 40#p418740

martin manning wrote: ↑Tue Sep 30, 2025 5:27 pm ... Did you measure the impedance ratio of the separate winding by itself?

That would be interesting.

I think adding a gridstopper to that fixed bias cathodyne should have the same effect / benefit as it would to the stock self biased version. Grid stoppers on the output valve control grids would probably be beneficial too. Merlin advocates for fixed bias etc in this rather more extensive cathodyne ...

.. The Thiele cab build does specify a 60 - 1600 Hz "usable range". I thought that was strange. ... A link to that info might be helpful. I suppose that frequency range refers to the cab being loaded with the driver it's been designed for. A different driver will almost certainly have different thi...

A problem with lifting the cathode etc is that a pretty common output valve failure mode has the HT somehow shorting to the heater.
So cutting the HT feed to the affected circuit might be better.

The output sockets on the amp's back panel should be wired in parallel, so electrically they're identical (apart from the shorting contacts on the main socket). So your idea should be fine. Running it with an 8 ohm load will change the amp's characteristics a bit, so your description of the change i...

That stage is inside the feedback loop. There's hardly any negative feedback, so it almost certainly doesn't matter, but as a general principle, it's not good practice to randomly introduce local gain changes / phase shifts etc inside a global feedback loop. Especially as that design architecture al...

... Used a hammond organ in series with another for the proper HT, was feeling myself. I don't understand what is meant by any of that? Got confident added a LFB on a single valve Please define unusual acronyms, what is a 'LFB'? During that break a White Claw got spilled on the TF and TF chassis du...

Anode current is determined by the relative magnitudes of the control grid and screen grid voltages. Have a good look at the 'average plate characteristics ' charts on p4 (screen grid at 250V) and p7 (screen grid at 400V). For a given anode and control grid voltages, consider how much higher anode c...

Have you verified that the feedback loop is negative? eg opening the loop increases gain.
Granted it would be unlucky to get it wrong with 2 different OTs.
It's probably best to leave the heater of the unused triode disconnected.

^^^ Wouldn't the issue be dependent on what is plugged into the FX loop? A piece of rack gear would probably prefer a low impedance signal but pedals are usually getting a direct signal from the instrument at a similar high impedance. Not as big a deal..? Your point also reminded me of Martin's twe...

... Each of the independent FX loops uses a stereo insert cable, with tip carrying send and ring carrying return to each of the grids of the phase inverter. If I make a cable with two TRS, one for each channel, join the tips with a 220k in series with each of the tips (summing the channels) to a mo...

... C-lator (a Dumble-style tube buffer/recovery unit). ... Do you realise that the 250k send loop level control in Dumble type loops makes the preceding send buffer somewhat pointless? I'm not saying they don't produce super awesome toanz, just that they aren't an example of a transparent fx loop,...