Half Power Switch in vintage Sunn Model T?

[SRT]

Hi gang!

I've done a search in the forums and haven't really found the info I'm looking for so I'm posing the question to the experts.

I've been running a '70s Sunn Model T for quite some time and have found that it's in dire need of a half-power switch.

Assuming that the "Hi-Lo Power" mod from Weber would work for me, how do I calculate which resistors to use?

Is there simply a spec on the 6550 data sheet I can use? Also, would I have to re-bias the amp after switching? For info, a PDF of the Model T schematic can be found here.

Ultimately, I'd love to use a pair of KT88s while on half-power for guitar but a quad of 6550s for bass.

Any help you could provide would be great.

Cheers,

--Sean

[Chuck H]

how do I calculate which resistors to use?

.

Your amp wouldn't use the resistors because it's fixed bias. In your case the "kit" would amount to nothing more than a switch that disconnects two of your output tube cathodes. You could get that switch from any Radio Shack. I would like to support Weber but their statement of "it can be used with fixed bias amps" when it's just a switch performing a simple function hardly makes it a "kit". You would not have to rebias for the switch. What you should do is compensate your speaker load to better match the OT secondary to the two instead of four tube setting. The "kit", as far as I can tell, doesn't provide for this.

You could actually use a 3PDT switch to add a big honkin resistor across the secondary when the "half power" mode is selected. This would be a more dramatic power decrease (since 100 to 50 watts isn't nearly as dramatic as you might think) and it would correct for speaker impedance.

Ultimately, I'd love to use a pair of KT88s while on half-power for guitar but a quad of 6550s for bass.

Unless you want to add two sockets to the amp you would spend a lot of time changing power tubes. With some effort you could set the amp up to run a pair of KT88's and a pair of 6550's simultaneously with only the KT88's in service at half power. This would mean building a second bias supply and remaining vigilant about 'which sockets take what tube'.

Chuck

[SRT]

Thanks Chuck!

The previous owner of the amp told me at some point that I could simply take out two of the 6550s, switch the output impedance switch to half of normal (ie. 4-ohm when plugged into an 8-ohm cab) and be OK. Is this along the same lines as what you've explained in your response? If so, I'll fetch a switch and install it. Too easy!

For my KT88 idea, I was thinking of replacing the 6550s in the 2-tube scenario above altogether with a pair of KT88s. I would assume a minimal change to the bias would be required but hopefully nothing more than a couple of resistors.

I rarely use the amp for bass any more (though it is a source of great joy when I use it in this capacity) so the full 150 watts is a tad much. I'm a sucker for output tube saturation and I love where this amp breaks up but it's simply FAR too loud for the stage. The sound guy usually just gives up and doesn't mic my cab altogether. Further, every attenuator I've used doesn't give me the same joy as the full blown-out 4 x 6550 tone (and I wouldn't expect it to).

Then again, growing up in the underground/indie scene where dueling half-stacks and SVTs abound has warped my tastes into the pantleg-flapping, spine-tingling tone-craver I am today.

Hmmm... maybe 4xKT88s is more up my alley after all...

Cheers,

--Sean

[DIY_Guy]

.You could actually use a 3PDT switch to add a big honkin resistor across the secondary when the "half power" mode is selected. This would be a more dramatic power decrease (since 100 to 50 watts isn't nearly as dramatic as you might think) and it would correct for speaker impedance.

Never really thought about the resistor idea. May have to try it. I'm a little confused as to how you would come up with the correct value resistor though. (I'm no good with math). And wouldn't the resistor only affect the one secondary tap you connect it to? I'm interested to to understand this a little better.

[Chuck H]

I've never done it either. Just something I thought of just then. And yes, the value would only be good for on ohm setting, UNLESS... You change the impedance switch in the amp to a DP. Then you could have the umpedance switch dictate which resistor and the half power switch dictate whether or not it's in the circuit. Or you could just use one "medium" resistor value switched in across the output jacks. With a resistive load it's not as critical as with a speaker load. Example, if an amp wants to see an 8 ohm load you CAN put an 8 ohm resistor in place of a speaker and the amp is perfectly happy. But a speaker has a complex impedance. Average speaker impedance might be something like 8 ohms up to 10 or 20hz then it ramps up as high as 100 ohms around 70hz, then it comes back down to 8 ohms at about 300hz and levels off to about 900hz and starts climbing again to some 30 or 40 ohms at 10 to 20khz. The "Ultimate Attenuator" is a re amp device that uses a 30 ohm resistive load for all amps at all settings and I don't hear anything about those units blowing up amps. Same with the Trainwreck/Dr.Z Airbrake, purely resistive load that averages some 20 ohms or so. You can't get away with this with a speaker because the impedance at the ramps is way too high at that point. But it seems that a resistive load can be fudged to good effect due to the very high impedance at some frequencies of an actual speaker. With this in mind, if you had say 4, 8, 16 taps, I think a single 16 ohm resistor could be used for all impedance settings. Though it would not give an equal amount of attenuation on all impedance settings, it should be safe.

Chuck

[SRT]

Hmmmm...

Chuck,

Thanks for the info. I'll keep the info regarding speaker impedance throughout the audio spectrum in my archive. Little tidbits like that help a lot!

I checked over the amp and found that there is, in fact, a bias pot as installed at the factory. This makes the amp the Model T (Super) as published in the schematics section of the Tube Amp Book (Aspen Pittman). If you don't have a copy, the only difference I can spot in the power supply is that R44 has been replaced with a 10K Linear Pot.

Now that I've done some reading (and along with your guidance), I've decided to just swap out the tubes for KT88s and figure out an attenuation solution (I've been reading up on just making one).

In a completely unrelated note, C19 and C20 in the schematic are both halves of a 80uf/80uf 450V canister. Do you think it would be safe to install a 100uf/100uf 500V part? I noticed that there is a bulge in the bottom of the cap and would like to resolve the issue before it blows.

Thanks for your help!

--Sean

[Chuck H]

If you do the "lift the cathode on two tubes" switch and simultaneously switch a 16 ohm 100 watt resistor (actually, use a pair of 33 ohm 50 watt resistors in parallel, they're much cheaper) you would get 60 watts at 4 ohms, 50 watts at 8 ohms and 40 watts at 16 ohms. Do the "resistors swapped by a second pole on the impedance switch" idea above and you would get 40 watts at all impedances. But you'd have to find room for eight resistors that get very warm. Maybe on top of the chassis with those aluminum housed jobbies.

The 100uf caps will be fine. Most of those cans are +/- 20% anyhow. So 100uf is actually still in spec for the amp. You won't hear any difference due to the cap value. You may hear a difference because the cap is new and working correctly though.

Chuck