using KT88 tubes

[Tone Junkie]

HBP here are a couple things Ive found Im thinking this power transformer and here is a KT88 power supply that could be usefull to copy.
Thanks Bill

[HotBluePlates]

Looks like somebody re-jiggered a Williamson for fixed-bias UL KT88's.

You can do the overly-elaborate biasing they use, but it just seems unnecessary. I might have to take that comment back if the source of the schematic had a well-reasoned idea to do it that way.

Also about the bias supply: they tried to take the Williamson cathode bias circuit (with bias adjust and balance) and turn it into a fixed-bias arrangement. I think they missed the mark and used way too many parts, and it would easier to have a simple Fender-style bias circuit with a pair of bias pots, one for each tube.

Other than that, the input stage is a simple volt-amp, the next stage is a split-load inverter, and the last pair (which you'd think was a long-tail inverter) is a differential amplifier to boost the signal form the split load to drive the output tubes.

I'd recommend not copying the feedback loop shown in that schematic, running from OT secondary back to input stage cathode; this requires an excellently-made OT designed with the Williamson in mind (to keep negative feedback from turning into positive feedback).

The 1kΩ screen resistors are probably not well thought-out, as the screen current is meant to contribute to the output power, and the largish screen resistors will tend to cause voltage drop and power loss in this circuit. About 100Ω right at the tube socket is the way to go.

The power supply ditched the 2 large chokes present in the original Williamson circuit, probably because of cost and weight (and because they used filter caps much larger than the Williamson).

In all, find a bass amp using KT88's, or the Major or other guitar amps referenced in the other thread. Copy those because they're known working designs. Make changes to the preamp as needed to voice the circuit for guitar.

[Willabe]

That's a KOC bias/balance from TUT series.


               Brad     

[sluckey]

This Dynaco circuit is a proven design. Sunn copied it exactly for their early bass and guitar amps. It's a clean, powerful amp.

[Tone Junkie]

I like a good amount of clean power. that should be fine . The P.transformer Im going to use will push 560v after the rectifier thats why i thought the whole for the plates then half for the screen voltage so would 560 for plates then 280 at the screens be a good match. i threw that other schematic up just to look at i didnt really understand what i had. Im looking at the sunn schematics. Slucky Ive never used a 6an8 tube is there an advantage over 12a_7 type tubes for the PI like in the dynaco schematic.
Thanks Bill

[sluckey]

Just another bird AFAIK.

[HotBluePlates]

That's a KOC bias/balance from TUT series.

Hmmm...  It really looks like a Williamson output stage, with the tube cathodes grounded and the Balance pot wiper connected to a bias supply instead of ground.

Williamson article

I think I'm seeing what I want to see, because the bias adjust and balance pots are higher values than the Williamson. I'll submit that the general approach may have been lifted from Williamson. I guess it's doable either way.

The rest of the circuit does in my opinion copy the Williamson, though you'll have to look at later versions to see the compensation cap in the feedback loop.

[Willabe]

I'm just talking about the bias circuit and it's parts values only.


               Brad