Vacuum Tube Harp Amps!

[fogtone noam]

newbie mod-ing sf super six (twin) for harp

Hi All

I am a harp player who has just started getting a little deeper 'into it' with the soldering iron.
I've been reading books on tube amps, done some upgrade mods on my stereo tube amp and would now like to experiment mod-ing my 100 watt silver-face super-six (twin) chassis -to optimize it for harp.

Specifically I would like to try changing to cathode bias and perhaps try different (lower) plate voltages. G. Weber suggests a 600 ohm resistor and a 25-100 uf cap per each output pair. He also suggests connecting the bias resistors to ground after taking them out of the normal bias circuit.

I was wondering what people here thought about the above. Can I not just cap off the bias resistors after disconnecting ? Are the above values correct? Also I would like recommendations re. resistor values for the pre-amp tube plates in order to lower voltage .

thanks in advance - Noam

[MWJB]

Hi Noam,

If converting to cathode bias I'd recommend that you use a cathode/fixed bias switch, see londonpower.com, search under Kevin's FAQ for the schemematic. There may be some situations where the harder dynamics of fixed bias are desired. In cathode bias mode try it both with and without the cathode bypass cap per pair.

Keep the resistors at the preamp plates at 100K, it's the power supply resistor that feeds these that you want to change, maybe try 56K- 100K at 3W. I'd shoot for around 160vdc at the 12AX7 preamp tube plates (V1 or V2).

Increase cathode bypass caps in the preamp to 100uf, increase coupling caps at preamp & PI to 0.1uf. Use a 5.6meg resistor at the input jack instead of 1Meg. Experiment with bypassing the 47ohm or 100ohm NFB load resistor with a 1uf cap (gives "presence").

Your amp should be serviced (new filter caps, bias supply caps & cathode bypass caps to ensure proper operation).

Bias - in fixed bias try 15-20mA, I like Sovtek 5881 in this kind of amp. In cathode bias you may want to see around 50mA per tube, 600ohms sounds about right.

[MWJB]

Further on cathode resistor values...

Exact value will depend on your plate voltages, there was quite a margin for plate voltages accross SF amps - 600ohms would be OK for B+ over about 480v, if voltages are nearer 400-430v try 330ohms, 470ohms for voltages betwen 430-479? Plate voltages suggested are measured to ground, without subtracting cathode voltage.

If you get a little over 50mA I wouldn't be too concerned, you'll lose maybe 40-45v accross the cathode, so even if you had say 510v at the plates, 510-45 = 465v, 465v*.050 = 23W dissipation.

[fogtone noam]

Thanks MWJB, The details are much appreciated. I will take a look at Kevin's faq for making this option switchable.