40 watt Sunn amp sounds great but lacks power

[Bob Opera]

I have two 40-watt Sunn amps, a Sonaro and a Solarus. They share the same 2xEL34 power amp while having different preamps (tremolo and reverb on the latter).

The Sonaro sounds great, however when dimed it is barely too loud for my apartment. My strat can almost make it break up if I dig into it. The Solarus, like any tube amp of this wattage, gets too loud at 2 on the volume and breaks up past 5-6.

I've tried swapping all of the tubes. I've measured 470V/38ma on the power tubes, checked all the resistor values and replaced one that drifted but to no avail. How should I tackle this?

http://www.prowessamplifiers.com/sch..._sonic_i40.pdf

Thanks!

[Gtr_tech]

Check the screens first. There's a resistor in the screen supply that commonly fails and causes low output. It should be across one of the. filter caps. Replace the caps as long as yer in there....they need it.

[Bob Opera]

Check the screens first. There's a resistor in the screen supply that commonly fails and causes low output. It should be across one of the. filter caps. Replace the caps as long as yer in there....they need it.

Thanks for replying. I've retested the resistors and they seem to be on spec. The ones across the filter caps are 22k and 47k respectively. The one going to pins 9 and 2 of the 7199 tube is 1 meg (out of circuit)



The filter can was replaced about 6-7 years ago. The tech used a couple 350V 33uf caps which so far have held up. I'll replace them with a 30/20/20/20 can to make it cleaner looking.



That 10 ohm resistor bottom-left was also added by the tech


This is what it looked like before being serviced.

[Gtr_tech]

Well then....I guess check all plate load resistors and check for DC leakage on any coupling caps. Did it ever work right after it was worked on?

[Bob Opera]

Well then....I guess check all plate load resistors and check for DC leakage on any coupling caps. Did it ever work right after it was worked on?

Will do, thanks! I don't think it ever worked right, though I have used it very seldom. It was never able to keep up with a drumkit so I bought a bigger Ampeg V4 and it's sat mostly unused since. I just shrugged it off to it being an underpowered bass amp. However, after stumbling upon the Solarus it became evident that something's not right. I've just never encountered an amp with such weak but undistorted, great sounding output.

Dumb question: Is there a way to tackle this by measuring the AC voltage on the primary of the OT, comparing it with the Solarus, and in the case of a discrepancy working my way backwards?

[gbono]

I'm working on a Sonaro and this one has the 16 and 8ohm taps used on the OT. If I use the 16 ohm tap the amp should put out 25Vrms for 40W. Did you measure and compare the output of your amps using a resistive load?

The 7199 in this amp is tired and measures low on a Hickock tester. I will rewire the socket for a 6AN8 since 7199 are only available as NOS and I can replace the output tubes for the cost of a NOS 7199. BTW the 7199 is also microphonic.

You mention that the PS filter caps are rated at 350V - I measure almost 500V (120V mains) on the plates of the output tubes on this amp.

[52 Bill]

Your amp doesn't seem to exactly match any of the schematics, there are bits that match one and bits that match another. The photos show a lot of things changed or modded, even the original photo of the chassis seems to be worked on.

There are a few things that I'd suggest, first being to remove and correctly replace the cluster of filter caps and dropping resistors. They look like an accident waiting to happen.

Then find a schematic that comes closest to your amp and return the amp to stock values and circuitry. I don't know why there are resistors added between pins 1 & 8 of the EL34s and I don't know why the negative feedback resistor was changed to a lower value reducing the power output of the amp. There also seems to be a missing cathode bypass cap in the input stage, but Sunn circuits didn't always have these caps.

[Bob Opera]

52 Bill, you are right. There are some clipped leads that are clearly visible even in the "before" pick.

Gbono, I mentioned the 350V 33uf caps because they are not rated high enough for the voltages in this amp, thankfully they haven't yet blown up. That tech certainly does beautiful work Would you mind posting some gut shots of your sonaro, if it's unmolested?

Thanks for all your suggestions, will try them out tonight.

[52 Bill]

...I mentioned the 350V 33uf caps because they are not rated high enough for the voltages in this amp, thankfully they haven't yet blown up.

I can't tell which caps are rated at 350V, but in the far end of the power supply as long as the tubes remain in their sockets, the voltages will probably stay below 350V. But pull the preamp tubes and watch out.

Like I said an accident waiting to happen.

Another thing that I noticed was that the bias supply divider has been grounded through one of the added resistors on the power tube socket. Be sure to fix this.

[Gtr_tech]

I redid a Solarus PI some years ago with a 12at7 LTP with great results.

[loudthud]

Another thing that I noticed was that the bias supply divider has been grounded through one of the added resistors on the power tube socket. Be sure to fix this.

That's normal for these amps.

What's not normal is the 5W resisitor mounted to the terminal strip under the output transformer. Is that 10 ohms? That should be 1K 2Watt IIRC. Also, check the speaker jack that switches the transformer tap. The contacts can get corroded and not make good contact.

[52 Bill]

That's normal for these amps.

Wouldn't that put a small negative voltage on one of the two output tubes?

[loudthud]

Wouldn't that put a small negative voltage on one of the two output tubes?

That resistor goes to pins 1 and 8 which are grounded. I like to move all the wires to pin 1 and connect a 1 ohm resistor between 1 and 8 so I can measure cathode current. The circuit uses the bias pot as a voltage divider, unlike most guitar amps. Look at the schematic linked above.

[52 Bill]

That resistor goes to pins 1 and 8 which are grounded. I like to move all the wires to pin 1 and connect a 1 ohm resistor between 1 and 8 so I can measure cathode current. The circuit uses the bias pot as a voltage divider, unlike most guitar amps. Look at the schematic linked above.

If you look at the photos, there is a resistor, maybe 330 ohms, between pins 1 and 8 on both power tube sockets. Pin 8 on both sockets are grounded. The 10K from the bias supply is connected to pin 1 on one of the power tube sockets, which elevates it from ground by 330 ohm and places a small negative voltage on grid 3 of that EL34. I can't believe that this is normal.

[Bob Opera]

That's normal for these amps.

What's not normal is the 5W resisitor mounted to the terminal strip under the output transformer. Is that 10 ohms? That should be 1K 2Watt IIRC. Also, check the speaker jack that switches the transformer tap. The contacts can get corroded and not make good contact.

So I replaced the 10 ohm resistor with a 1K and the power is back! Thank you! What was the tech possibly thinking when he added that 10 ohm resistor? Would a 1/2 watt 1k resistor do? I only have 1/2 watt and 10 watt versions on hand. My Solarus has a small carbon comp 1/2 watt-looking 1k resistor.

While waiting for the filter cap can to arrive, should I get rid of those 330 ohm resistors between pins 1 and 8 of the power tubes? On the Solarus pins 1 and 8 are connected.

Thanks all for your invaluable input.