I’ve been away, advise requested, rehearsal report added.

[Charlie Wilson]

Sounds WAY better, both in clean and OD. I don't know what power tubes you have but I find with new production you have to bias them up a bit to fill out the mids. All of the real Dumbles I have had my hands on were biased pretty warm by him.
CW

[donvan]

All of the real Dumbles I have had my hands on were biased pretty warm by him.
CW

At what percentage did he tend to bias amps?

[Bombacaototal]

I was always curious about this too too, maybe which mA for which plate voltage is always an easier reference.

[talbany]

Dumble ran ODS 121 at an idle current setting of 125 mA (.125v) for a pair of GE6550A. (see pic below)
With 121's plate voltage of around 540V and an idle current of 62.5 mA plate dissipation would be 33.75 W (A plate dissipation value of 42 W is specified) and 80.36% of the maximum plate dissipation for a GE-6550
So Dumble didn't always adhere to the 70% Max dissipation rule for Fixed bias P/P

Dumble Overdrive 150 Special #121 012.jpg

That's Hot
Tony

[Bob-I]

Sounds WAY better, both in clean and OD. I don't know what power tubes you have but I find with new production you have to bias them up a bit to fill out the mids. All of the real Dumbles I have had my hands on were biased pretty warm by him.
CW

I originally built this amp for 6V6s, deluxe iron, but I’ve found I like the way 6L6s sound. Even at low voltages, I think about 380v, they sound richer. Right now I have a pair of TADs that are maybe 10-12 years old, but very little use in them. I have them biased at just over 50ma, so almost 70%. I went warmer but didn’t notice a difference so I’m sticking with this for now.

[Bombacaototal]

Dumble ran ODS 121 at an idle current setting of 125 mA (.125v) for a pair of GE6550A. (see pic below)
With 121's plate voltage of around 540V and an idle current of 62.5 mA plate dissipation would be 33.75 W (A plate dissipation value of 42 W is specified) and 80.36% of the maximum plate dissipation for a GE-6550
So Dumble didn't always adhere to the 70% Max dissipation rule for Fixed bias P/P
Dumble Overdrive 150 Special #121 012.jpg
That's Hot
Tony

That's fantastic input Tony. I had never paid much attention into that figure printed on that amp. Is 540V plate to cathode or plate to ground (doesnt make a massive difference, I know)?

I was actually chatting to Ryan (thetonegeek) recently on 002 and its original fixed bias, and I think that amp lives on the polar opposite side of the bias. With 435V plate to cathode (assumed, but by converting voltages from JP schematic [100V] to USA [117V] yield in a lower figure), with the 3.3K resistor and a non-high gain 6L6 that amp is probably just shy of 50%. Although its a different CF to 121 with different operating voltages and all, it is still interesting as a comparison.

An assumption I have is that maybe high gain pre amps (like 002) had low bias, and maybe low gain pre had a high bias (? - not entirely sure on the gain assortment on 121). At least 002 biased hot is fairly constipated, stingy, and too gainy for me...

[CHIP]

As I recall, although I may be wrong , but I think #183 was biased on the cold side.

[Bob-I]

As I recall, although I may be wrong , but I think #183 was biased on the cold side.

Scott Lerner suggested bias at about 35ma, but I’m not sure what amp he was talking about. I run my 4x6L6 blues master at 52ma. Unfortunately I can’t ever crank that amp up, which is why I built the little one.

[Charlie Wilson]

Yeah I think the EL34s sound nice biased a little cooler. The 6l6s in the real Dumbles I had my hands on were 45mA or higher.
CW

[talbany]

183 was biased @ 30.3 mA @465 with a pair of Siemens EL-34's IIRC (37mA would be 70%)

BTW. I put it right on the layout
https://ampgarage.com/forum/download/fi ... &mode=view

Is 540V plate to cathode or plate to ground

PT is a Triad R-25A.(Same as in #009 D.L)Should deliver around 530 VDC plate to gnd under load &(bias depending)

o06000450106438998221.jpg

121_009.jpg

Tony

[Bombacaototal]

Yeah I think the EL34s sound nice biased a little cooler. The 6l6s in the real Dumbles I had my hands on were 45mA or higher.
CW

That is really interesting Charlie. Were the 6L6 amps ODS? Its definately much hotter than I have run mine so far.

Thanks for the input on 183 Tony, on the cold side for sure