Tube Preamp with tube phono section - tube Q's

[kach22i]

I own a tube preamp that I bought used in late 1990's.

Have replaced four E88cc (6922) tubes about every 8-years, never replaced the four PC900's in the med/low phono section (that I can recall).

The replacement tubes have been two in the main line section, and two in the phono hi-gain section.

I have never experienced noise/grain only excessive heat, loss of both channels or weak channels in either R&L that migrate randomly between R&L prior to replacement.

These failings happen in both CD and LP playback, not just one or the other. In other words, in the line stage.

I have always used the medium phono gain at 47k for MM cartridge. Never used a MC and do not plan to.

Question: from the diagram in lower left, the two tubes labeled "phono hi-gain", do you think these are in any way active when using a MM?

Reasoning: if these tubes have never been used then I perhaps can rotate them into the line-stage section, right?

EDIT: the price of these small preamp tubes is kind of crazy, and I do not have a tube tester. Just looking to be frugal but safe.

 

[Wes]

bought used in the late 1990's sez cap. rot to me

 

[JeffS7444]

The manual says that the 6922s are part of the RIAA circuit, so you need those for all phono, and very likely, you need all PC900s in place as well as well: Circuit description suggests to me that selecting low phono gain is not bypassing "optional" gain stages, but simply altering the gain of the same circuit. I'm kind of bothered by apparent scorched areas of transformer wrapping but that may not be your immediate problem.

BTW, when you lose signal, what happens if you jiggle RCA connectors and twirl some of the controls?

 

[kach22i]

The manual says that the 6922s are part of the RIAA circuit, so you need those for all phono, and very likely, you need all PC900s in place as well as well: Circuit description suggests to me that selecting low phono gain is not bypassing "optional" gain stages, but simply altering the gain of the same circuit. I'm kind of bothered by apparent scorched areas of transformer wrapping but that may not be your immediate problem.

BTW, when you lose signal, what happens if you jiggle RCA connectors and twirl some of the controls?

Thank you for all the information, spot on to the question at hand.

All RCA connections and speaker connections were cleaned and treated in my system about 3-weeks ago, I forgot to wear knee pads and still have not 100 percent recovered. Sucks getting old.

No loose connections, and I am careful not to mess with things until equipment is cold and dormant.

Last week I look the cover off and spray cleaned the board and tube pins, hence the recent photos posted.

Just a little dust, a few cobwebs and that transformer discoloring to note.

I used a little lube for plastic on balance and volume controls.

Controls seemed to really snap and click with authority and turn smooth as silk in the process

Additional information: a little over a month ago I noticed some channel imbalance. I attributed it to room acoustics and treatments that I had been altering every few days.

The speaker R&L imbalance would come and go and was not drastic.

Was always the left channel being stronger, right channel being weaker.

My solid state stereo amp stopped working about 1-1/2 weeks ago.

I replaced it with two bridgable SS amps.

The channel imbalance which varies slightly in severity over several hours of play was still there but not at first, it took a day or two to show back up.

When it did show back up it was mirror image of before. This time the right channel was stronger and left channel weaker.

This is when I ordered 4 matched 6922's, should be shipped soon.

No small detail that I lost an 30 year old SS amp in the course of this.

However I only wanted to understand my circuit design better, and for that I thank you.

If the new tubes do not fix the issue I will get the preamp serviced by a professional plus get my SS stereo amp repaired.

This Italian made preamp has only had to be in the shop once before. Circumstances were it was running hot, inlaws came over for a visit and I had music playing at low level. My mother inlaw dropped a large heavy photo album about foot in front of my audio rack and the music stopped.

My local hi-end shop where I purchased it from shipped it from Michigan to Ohio, it came back running cool to the touch and sounding good. This was 16 years ago. I think all they did was replace the (4) 6922's.

About 8 years ago I replaced the same 4 tubes.

The preamp is running too warm for my taste again, sure sign to me those tubes are begging for replacement.

From what I've been reading about other tube preamps need new tubes every 3 months, once a year or if lucky every 3 years, I think I'm blessed with a pre that seems to be on 8 year intervals.

And those other preamps tend to take out the nearest resistor or capacitor in the process of tube failure - no thanks

 

[Pete G]

For what's it's worth, rolling these drive tubes is fairly inexpensive. I stay away from NOS. Current production from JJ, EH, Gold Lion, Shug or Tung is reasonable on the pocketbook. After a burn in period to polish, these things may last for years even with daily use. Power tubes, EL34, KT88, etc don't last as long; at least with the frequency I run my system, which is daily!

 

[kach22i]

For what's it's worth, rolling these drive tubes is fairly inexpensive. I stay away from NOS. Current production from JJ, EH, Gold Lion, Shug or Tung is reasonable on the pocketbook. After a burn in period to polish, these things may last for years even with daily use. Power tubes, EL34, KT88, etc don't last as long; at least with the frequency I run my system, which is daily!

The shop put in JJ's 16 years ago, then 8 years ago I put in NOS Hungarian Tungsram.

I stopped playing regularly 20013-2019 hardly at all, then four months ago hooked up in new house and both my wife and I were blown away by the sound. A different class than what we remembered, but new room so who can say it was the tubes fully broken in and getting sweet.

I ordered NOS Reflectors (Russian) mostly because they were top rated at thetubestore (my usual source), but ordered them for 40 percent less thru Viva.

I might try top of the line new tubes from EH next time. Hours wise the Tungsram should have lasted longer in my opinion, but I've been playing them often, long and hard these last four months.

Thanks for the input.