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Old 15th June 2014, 06:41 PM   #21
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Update.

I've replaced the entire Pcb and all components except for the coupling caps. Still hissing although not as bad. Balls...
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Old 15th June 2014, 08:38 PM   #22
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I should add, re grounding I have the right channel input grounded to the star ground but not the left. Both negative speaker posts are grounded to the same star ground.
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Old 16th June 2014, 01:12 AM   #23
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How about your speakers - have you tried them with a different amp to see if there's not something wrong with them.

Sans speakers I'd swap in some different coupling caps - get some Orange Drop 716P they are a dollar or 2 (quid or 2), easy enough....

Cheers,
Bob
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Old 16th June 2014, 04:50 AM   #24
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I've tried swapping the speakers fr left to right and with a different set, no change.

I've also swapped the caps over toobut the nouse remained on the left channel.

I've an SE build on the go atm, i'll steal the caps from that and see how it goes.
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Old 16th June 2014, 10:29 AM   #25
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I've tried more.

I've grounded both input rc's - no change.

I've temporarily swapped the opt's around - no change.

I've swapped the coupling caps around - no change.

What else is there? I've changed the volume pot already. All that's left is rca sockets, speaker terminals and some wiring.
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Old 22nd June 2014, 02:04 AM   #26
cjkpkg is offline cjkpkg  United States
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Is the hiss the same when a source is plugged in or not?

How about no source and inputs shorted to ground?
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Old 22nd June 2014, 11:47 AM   #27
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Source makes no difference. I've not tried shorting them, i'll give it a try.
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Old 22nd June 2014, 01:31 PM   #28
cjkpkg is offline cjkpkg  United States
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If it goes silent on short then its your source.

If not then you probably have either a bad cap or noisy resistor. I had a devil of a time with some 6cg7 preamp tubes...I wish I could say what the problem was but at higher bias levels some hissed like crazy and some were dead silent.

You might try biasing down the front end to 5mA or so and see if running the front end cooler helps.

I believe its a front end issue.

Remove the 12at7 and fire it up to verify...if its silent then its a front end problem for sure.
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Old 22nd June 2014, 02:11 PM   #29
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Sorry for my inattention. I am in the process of moving all my stuff about 1200 miles. We had a plan that involved a timely organized move spread out over 4 months and several smallish loads. Part of this was predicated on the fact that the temporary housing we are moving into is still occupied. We put our house in Florida on the market in June with the expectation of leaving in August. A buyer was found almost immediately, that unfortunately wants us out on June 30. We are rather busy! Full scale panic is more like it.

It seems that nearly all of the things that I would expect to cause a hiss have been ruled out by substitution. There are a few more things that can be tried.

The most common cause of abnormal hiss is a resistor, tube, or capacitor, in that order. It seems that you have changed all of them.

You state that the amp worked good for several months before developing the hiss. Did you change anything around the time that the hiss appeared.

Does the amp hiss at all with the 12AT7 removed. If so the problem is in the output stage (unlikely but worth a try)?

The fact that the hiss vanishes when the power is shut off is a clue, but to what??????

Hiss is usually caused by a component becoming noisy, often due to a manufacturing defect forcing the majority of the internal current flow to go through a small area. A resistor with bad laser trimming, or a capacitor with a thin spot in the dielectric or poorly made contacts can cause hiss.

Hiss can also be caused by a tube oscillating in the RF region while carrying audio. This has been an issue in the TSE with some 5842's but hasn't been seen in an SSE....but there is always a first time.

Do you have a supplemental power supply capacitor? If so, try disconnecting it. I would expect it to affect both channels, but you never know. Also try disconnecting the choke if you have one, substitute a 150 ohm 5 watts resistor, or even a short if you don't have a resistor (expect some hum, but listen for the hiss).

Try a different rectifier tube, it could be breaking down due to gas. (blue glow around the elements?)

Do you have access to a variac? If so turn the line voltage down about 20% below normal and bring it up slowly. Does the hiss remain constant, or slowly increase with voltage? If so this is a noisy component. Is the hiss absent, and appears instantly at a particular voltage, or go up, then down, or generally change pitch randomly with voltage change? If so this could be oscillation.

You mentioned that the his can be brought on by playing with the volume control. Once the his appears, is it constant, or can the pitch or level of the hiss be changed with the volume control. If so, this sounds like oscillation. Can the hiss be brought on or changed by running your finger around the glass on the 12AT7's envelope? If so this IS oscillation.

Bring a portable radio or TV near the SSE. Try all available frequency bands on the radio or TV. Try the finger on the 12AT7 glass. Does the SSE ever interfere with radio or TV reception when the hiss appears? If so this IS oscillation.

Does the hiss appear with the input wiring disconnected from the PC board? Can it be provoked with a finger on the glass of the 12AT7 and the wiring disconnected? If not, there may be some RF resonance in your input wiring that excites an oscillation in the 12AT7. If this is the case the resonance needs to be moved to a different frequency that the 12AT7 doesn't respond to. This is usually done by changing the LENGTH of the wiring, or the TYPE of wiring used, or both. Try twisted pair instead of shielded cable. Try untwisting your shielded cable. Try disconnecting the ground on the affected channel. Try swapping the connections of the two channels at the board, and at the input connectors, or relabeling the connectors (left becomes right....)

You have tried several 12AT7's. Are they all JJ's? The JJ 12AT7 is made with a different plate structure that any other 12AT7 I have seen. SSE and SPP users seem to like them, but they are different. Try another brand if this is the case.

The SSE uses grid stopper resistors on the 12AT7 to prevent oscillation (R12 and R22). A simple metal film resistor has been effective at preventing oscillation in all SSE amps so far, but this should ideally be a carbon composition resistor. The value isn't critical, anything from 1K to 5.1K should work. I switched to 4.7K carbon comp on the TSE several years ago to kill stubborn oscillation on some 5842 tubes. The 5842 is MUCH more prone to oscillation than the 12AT7.
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Old 23rd June 2014, 05:49 PM   #30
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Hi George, No worries, I hope your move goes well. I really appreciate you taking time out to reply.

I've started working through your suggestions.

No changes were made whic then resulted in the noise. It just started one day listening to a film. I have since the noise started removed an input selector so there is only one set of rca's connected directly to the volume pot although that was some time ago and may now difference to the noise.

I've tried three gz34's now but with no change. There is a nos tube in there atm.

Re the volume exciteing the noise, I tried recording it. Initially the input was disconnected then I turn it off, plug the input back in and turn it on again. I give the volume control some stick as well which you can just hear. Apologies for the quality, my phone doesn't pick it up very well and adds its own noise to the recording. The hiss is a lot louder than the video suggests. Its like a very old record.

Click the image to open in full size.

Touching the glass makes zero difference to the noise and a radio nearby is unaffected by the amp.

The 12at7's are a telefunken, a mullard and some old nasty worn out pos, no JJ's. All made no difference.

Re the input wiring. I've had to use quite a long length. Its twisted pairs, both channels and shielded. I've grounded the shield to the star ground for the amp. Its the purple cable in the pic.

Click the image to open in full size.

I haven't tried taking the 12at7 out yet. I'll do that in a bit and report back.

Thanks again
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