I have a Lamm preamp in need of repair...

[microstrip]

If its in both channels it could be a regulator that is oscillating. That can happen just before the regulating transistor goes south entirely.

I could not see any series transistor or regulator in the photos - just what looks to be a TO3 filament regulator.

 

[Atmasphere]

I could not see any series transistor or regulator in the photos - just what looks to be a TO3 filament regulator.

View attachment 162439

Hm. Not a lot to this circuit!
If the filament regulator were oscillating, that would do it.
The photo is blurry but it looks like a tube off to the left. I'm doubting its in the audio path, so I presume it has a power supply function. Might just be a rectifier. If so the idea that the filament supply is oscillating is more likely.

 

[DasguteOhr]

The LL2.1 uses one 12AU7 and one 6DJ8 per channel, does not invert phase and has a low output impedance. I would think about a topology similar to the old Audio Research SP8 - two triodes in series with a cathode follower.

The SP8 was known to have such problem - the 6DJ8 is prone to hiss. My original SP10 was a nightmare in this aspect, fortunately it was solved in the SP10 mk2 and I upgraded the circuit.

But I am just guessing!

that's deceptive, the preamp has everything in it. ecc82 parallel ,the ecc88 is srrp upper cathode output with frequency-dependent negative feedback. regulator, Ufk and the feedback can be made such problems.

 

[Atmasphere]

that's deceptive, the preamp has everything in it. ecc82 parallel ,the ecc88 is srrp upper cathode output with frequency-dependent negative feedback. regulator, Ufk and the feedback can be made such problems.

For it to be a tube it would have to hit both channels at once. It can happen but its not likely. Without seeing it, its all conjecture.

 

[DasguteOhr]

For it to be a tube it would have to hit both channels at once. It can happen but its not likely. Without seeing it, its all conjecture.

I described it in the other thread. I would first measure anode, heating voltages of ecc 88 to rule out this particular error in the SRRP circuit. Post#11

Lamm LL2.1 Lover Looking for replacement suggestion

I'm tired of waiting for Mrs Lamm to set up some sort of repair shop so I'm buying a new preamp- spending $5000 or so on the used market. Any recommendations for a tube preamp that will make me just as happy?

www.whatsbestforum.com

I once had a preamp where a resistor was broken and it caused a similar error pattern

 

[Atmasphere]

I described it in the other thread. I would first measure anode, heating voltages of ecc 88 to rule out this particular error in the SRRP circuit. Post#11

Lamm LL2.1 Lover Looking for replacement suggestion

I'm tired of waiting for Mrs Lamm to set up some sort of repair shop so I'm buying a new preamp- spending $5000 or so on the used market. Any recommendations for a tube preamp that will make me just as happy?

www.whatsbestforum.com

I once had a preamp where a resistor was broken and it caused a similar error pattern

OK I took a look.

I've seen tubes with the heater/cathode limit exceeded. Generally I would expect a tube like that to even be working. If there is noise in the filament supply it would express that though.

If this preamp has a design problem where the filament/cathode limit is exceeded on the top tube then that would explain both channels being noisy.

 

[tima]

Might just be a rectifier.

6202 voltage rectifier

 

[DasguteOhr]

OK I took a look.

I've seen tubes with the heater/cathode limit exceeded. Generally I would expect a tube like that to even be working. If there is noise in the filament supply it would express that though.

If this preamp has a design problem where the filament/cathode limit is exceeded on the top tube then that would explain both channels being noisy.

I don't think this is a design flaw, but a resistor that is no longer have his value can cause the tubes to slowly become defective. It's a gradual process that keeps getting worse. I would measure the voltages to rule out the error.

 

[Alan Nordoc]

Hm. Not a lot to this circuit!
If the filament regulator were oscillating, that would do it.
The photo is blurry but it looks like a tube off to the left. I'm doubting its in the audio path, so I presume it has a power supply function. Might just be a rectifier. If so the idea that the filament supply is oscillating is more likely.

I'm going to give all this info to the tech I use and have him take a look. Thanks so much. I'll report back.

 

[Atmasphere]

I don't think this is a design flaw, but a resistor that is no longer have his value can cause the tubes to slowly become defective. It's a gradual process that keeps getting worse. I would measure the voltages to rule out the error.

Usually that sort of thing is something a carbon composition resistor does. I'm not used to seeing metal films do that!