Sunn Sonaro weak output

[dwhutchens]

This amp was previously used by a marching band to amplify a piano bass and was ran off a car battery and a power inverter. It was discarded after the power tubes were broken in transit and the electronics class was unable to repair it after replaced a couple of sets of tubes. My brother brought it to me and after replacing the filter caps and GZ34 rectifier it is working. With a cheap Yamaha bass it sounds ok on the low notes but weaker on the higher strings but is not as loud as I think it should be. It sounds like crap with a guitar. Not much clean volume and nasty distortion. The only schematic I can find calls for EL34 output tubes and a 7199 phase inverter. This amp has 6550s and a 6an8a pi. I put a 600 hz 150mv peak to peak signal into input 1 and I start to get clipping at pin 9 of the pi at around 7 to 7.5 on the volume setting. there is no clipping previously in the circuit. The power supply rail is high compaired to the schematic. 545v before the choke, 536 after the choke, 453 after the 22k, 339 after the 47k, 348 on pin 1 of the pi and 89 on pin 2. If anyone has any experiance on this amp and could help point me in the right direction I would appreciate it.
Thanks for taking the time to read this,
David

[loudthud]

Go to the link below and look for a schematic under the Schematic Diagram tab or the Service Manual tab.

http://sunn.ampage.org/site/

Voltages sound about right for today's line voltages. Check voltages around the phase inverter. Do you have any way to measure the bias current in the 6550's ?

Check the speaker jack that switches the transformer tap. These don't switch well after 30 years. I prefer to install a toggle switch.

[Amp Kat]

What kind of 6AN8 is it ? I think that was supposed to be KT-88's so Louthud may be correct with the bias issues and thinking tube currents. Sounds like your getting crossover distortion IMO.

[Enzo]

SInce the tubes were shattered, I would not assume all the support circuitry was working. Verify that there is B+ voltage on all pins 3 AND pins 4 of the power tubes. verify that the bias voltage on pins 5 are about the same and at a likely level. My first reaction would be that one side was not amplifying in the push pull. I too wonder what current is flowing in those tubes.

All the Sunn amps of that era were similar inside. Sunn always used the 7199 PI with EL34s and the 6AN8 PI with the KT88s. As far as I know, the Sonaro would have had the EL34/7199. The OT would be 3-40T and the PT would be 3-4PT. If your transformers are not those nubers, your Sonaro is likely one of their other models hinding inside it.

Do the voltages around teh PI tube come close to those on the schematic?

[dwhutchens]

More info

Thanks guys for the responces. I dug up a schematic for a Sorado amp that calls for KT88s and a 6AN8 PI tube. The voltages on this schematic are much closer to what I am measuring on my amp. To answer some of the questions you guys have here goes:

I replaced the speaker jacks and did away with the switching jack when I replaced the filters

voltages on the PI are; pin 1 = 348, pin 2 = 90, pin 3 = 106, pin 6 = 90,
pin 7 = 36, pin 8 = 0 and pin 9 = .86

the tube brands are: Phillips/ECG gold base 6550 and Sylvania USA 6AN8A

I cant see a crossover notch at any bias setting between -50 and -77 volts
until after the waveform starts clipping but I think that is because the PI is clipping too soon

I moved all the grounds to pin 1 of the 6550s and put a 1 ohm resistor between pins 1 and 8, with the bias set at -68v I get 46 ~ 47 millivolts across the resistors

I think the problem is with the PI but thats just a guess, could the .25 coupling caps between the PI and the 6550s be bad, any other ideas are welcome

Thanks again,
David

[Enzo]

Did you check the transformer numbers to see that they are the appropriate ones for these tubes?

[dwhutchens]

transformer #s

Sorry, I forget to check that. The numbers stamped on the transformers are as follows:

PT = 28-2360
682020

OT = 28-1360
606941

These numbers don't match anything on either the Sonaro or Sorado schematics I have but since my voltages much more closely match those on the Sorado schematic I assume at least the PT is for use with KT88/6550 tubes.

Thanks,
David