100 watt Super Bass with KT88’s

[pompeiisneaks]

Look at that belly! Sounds great man!

~Phil

[wpaulvogel]

Look at that belly! Sounds great man!

~Phil

[ViperDoc]

Great sound! Lots of cream in there.

[angelodp]

Very cool amp and the placement of the bias pot was an inspiration. I take it no issues at all with the longer wire runs for the bias pot? Any hope of seeing some shots of the potentiometer side of the amp. I think you did the mains switch for Live/Neutral ala TW. Where did you sneak those bias wires in? oh BTW sounds immense and true to form.

[angelodp]

It appears that you have a fender style 10k bias pot. The Super lead schematics show a 22k pot into a 47k resistor. I am thinking about implementing what you have done with the bias circuit and test points. Did you find a 25k fender style bias pot? Do 25k or 50k chassis mount bias pots exist? Am I better served on a 100w build with a 50k pot?

[wpaulvogel]

The pot is a 25k linear CTS pot just like the midrange pot. The wires go to the board at the original pot location. Red and black wire.

[angelodp]

Hi Paul, thanks for the info. I do see that 25k is the way to go on this build.

Best A

[angelodp]

Minor question if you can Paul. How do you keep the PVC wire jacket from shrinking back or cracking at the turrets. Your soldering is perfection? Are you using a special type of wire...Teflon coating?

I also see that you are forgoing the .68/2.7k on the V1b cathode in favor of a parallel cathode. I also see that you are not using the .0022uf cap and have a .02uf in its place. Clearly you favor this arrangement, the amp sounds amazing.

[pdf64]

This amp is biased at 69% of 42 watts (29 watts per tube). Plate voltage at 65mA per tube is 450 volts. I think it’s the output transformer impedance that limits the output power to 110-112 watts clean RMS (42.5 volts on a 16 ohm dummy load) with the KT88’s. Although they chugged up there to a maximum of 57.6 volts (208 watts). It’s an impressive amp.

Just to note that 42W seems to be the (usually unpublished) absolute max anode dissipation limit for KT88. The design max is 35W, so the design centre (ie by which an EL34’s 25W anode limit is derived) is probably about 30W, hence idling them in AB1 fixed bias above 21W might be compromising their operational life.

https://tubedata.altanatubes.com.br/she ... k/KT88.pdf

A cynic might think that MO / GEC (UK) fiddled their KT88 info to make them look competitive with GE’s (USA) 6550A, which has a 42W design max limit.
https://tubedata.altanatubes.com.br/she ... /6550A.pdf

[wpaulvogel]

Minor question if you can Paul. How do you keep the PVC wire jacket from shrinking back or cracking at the turrets. Your soldering is perfection? Are you using a special type of wire...Teflon coating?

I also see that you are forgoing the .68/2.7k on the V1b cathode in favor of a parallel cathode. I also see that you are not using the .0022uf cap and have a .02uf in its place. Clearly you favor this arrangement, the amp sounds amazing.

I use PVC 22AWG topcoat wire, the wire offered by Valvestorm and Triode I think it’s 1015. Just as the solder melts around the wire and turret, I set down the solder, remove the iron and place my fingertip on the insulation to act as a heat sink and to protect the insulation from deforming. I have thick skin and it usually doesn’t get me too hot. You might need a light glove if the heat bothers you. I’m afraid a glove might imprint the insulation though.

[wpaulvogel]

This amp is biased at 69% of 42 watts (29 watts per tube). Plate voltage at 65mA per tube is 450 volts. I think it’s the output transformer impedance that limits the output power to 110-112 watts clean RMS (42.5 volts on a 16 ohm dummy load) with the KT88’s. Although they chugged up there to a maximum of 57.6 volts (208 watts). It’s an impressive amp.

Just to note that 42W seems to be the (usually unpublished) absolute max anode dissipation limit for KT88. The design max is 35W, so the design centre (ie by which an EL34’s 25W anode limit is derived) is probably about 30W, hence idling them in AB1 fixed bias above 21W might be compromising their operational life.

https://tubedata.altanatubes.com.br/she ... k/KT88.pdf

A cynic might think that MO / GEC (UK) fiddled their KT88 info to make them look competitive with GE’s (USA) 6550A, which has a 42W design max limit.
https://tubedata.altanatubes.com.br/she ... /6550A.pdf

I warned the customer about the risks of the bias setting and he decided that he preferred the tone of the hot bias. Once he received the amp, he adjusted the bias cool to hot and found he preferred the 29 watt setting.

[ViperDoc]

Minor question if you can Paul. How do you keep the PVC wire jacket from shrinking back or cracking at the turrets. Your soldering is perfection? Are you using a special type of wire...Teflon coating?

I also see that you are forgoing the .68/2.7k on the V1b cathode in favor of a parallel cathode. I also see that you are not using the .0022uf cap and have a .02uf in its place. Clearly you favor this arrangement, the amp sounds amazing.

I use PVC 22AWG topcoat wire, the wire offered by Valvestorm and Triode I think it’s 1015. Just as the solder melts around the wire and turret, I set down the solder, remove the iron and place my fingertip on the insulation to act as a heat sink and to protect the insulation from deforming. I have thick skin and it usually doesn’t get me too hot. You might need a light glove if the heat bothers you. I’m afraid a glove might imprint the insulation though.

That Valvestorm 22AWG is what I used to wire up my latest two amps, it's great wire. Strips very easily, twists like a champ and doesn't fray when you treat the ending.

[harleytech]

So you can run KT88's in a 100 watt Super Lead as long as you replace the 220 resistors in the PI to 100K and change the bias resistor ?
The Stock 100 watt PT & OT should work and be ok ?

[wpaulvogel]

Yep, change to the 100k’s, the bias dropping resistor 27k was also reduced to 22k to center up the bias pot. The amp made about 112 watts clean rms and about 208 watts full power. The output transformer impedance restricts the power output.

[Mark]

The thing that I have against using KT-88’s in a Marshall is that they are typically biased quite cold and the transformers run quite hot. I don’t know if the tonal pay off is worth the stress to the transformers.