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Looking to get back into tubes-with 1625 output

Ran into a bug. The amps go into oscillation fairly quickly when bringing them up on the Variac. Did some digging over on the DIY site, and looks like I'll need to add plate stoppers and screen stoppers to stop the oscillation.

It looks like this is a fairly common occurrence with the 807/1625 tube.

Anyone else have this issue with these tubes?
 
Freo-1 said:
Ran into a bug. The amps go into oscillation fairly quickly when bringing them up on the Variac. Did some digging over on the DIY site, and looks like I'll need to add plate stoppers and screen stoppers to stop the oscillation.

It looks like this is a fairly common occurrence with the 807/1625 tube.

Anyone else have this issue with these tubes?
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Yes, and when they start to go they really go fast. What circuit are you using?
 
s-petersen said:
can you post a schematic,are you using NFB?
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The schematic is over at my buddy’s, as he has them on the workbench right now. I need to chase it down and scan it in. I got the original from him as a paper copy only, and did not get around to scanning it in.

The amp(s) has a pair of 12SN7’s for the input. They are configured in Tetrode mode, with NFB employed. It also is set up for fixed bias. From researching for this project, cathode bias and ultra linear did not seem to be a good idea for the 1625 tube. The plate voltage is around 500V.

I’ll get the schematic posted soon. Will try the anode and grid stoppers to see if that takes care of this.
 
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Freo-1 said:
From researching for this project, cathode bias and ultra linear did not seem to be a good idea for the 1625 tube. The plate voltage is around 500V.
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The screen is rated to only 350V, so some kind of regulation would be in order if using really high B+.

jeff
 
Oh yes, no question. There is seperate screen voltage supply and regulation that is 300 volts. According to my spec sheet, 300V is the max safe voltage for the screen grid.
 
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It's always good to get a second set of eyes when issues like this pop up. My buddy took the amp over to one off his good buddies, and lo and behold, a simple wiring problem was found and resolved. They checked it on the o-scope, and it all looked good. :thmbsp: It diid need the grid and anode stoppers.

So, the amps are getting burned in to make sure it survives infant mortality. I'll pick it up later in the week, and post some pictures.

I think if one wants to stay with tubes long term, this type of amp is a very good option. The tubes are high quality, plentiful, and best of all, inexpensive. It allows one to spend the cash on better parts instead.
 
Well, these guys have been burning in for several hours now, and all appears well. The bias is holding rock solid at 40 ma, with 500V on the plates, 310 volts on the screen. The amps are very quiet.

I was able to source some Canadian 12SX7 input tubes, along with 1940's Army Signal Corps 1625 tubes. It needs to break in (especially the Mundorf Silver/Gold coupling caps), but so far, sounds very nice.

I'll be quite happy NOT to have to use Chinese/Russian 6500/KT88 tubes anymore.
 

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After listening to this for a couple of days, I’m very happy with this. The 12SX7 is a fine tube, very quiet and holographic. The 1625's look very cool, and sound great.

In fact, I think I like this sound better than my old Citation II. It sure will be easier to keep these with reliable power tubes than the Citation. Just wish I had done this sooner!
 
Hi. New guy here. :D

Enjoyed your build report. Those old tubes were common in ham radios weren't they? How does the amp sound relative to more conventional hifi type tubes of the era and todays EL34 types? Worth experimenting with?

Nice look with the blue trafos. Enjoy.
 
Thanks, and yes, my buddy said this was the tube that helped won the war.

They are (IMHO) very similar sounding to a 6L6 tube, only with more drive. I’m running the plate voltage at 500V, but one can safely run them at 600V. The one thing to look out for is the screens, as they should not be run above 300V (within 10%). So, if one decides to run ultra linear, the safe outpt power will be less.

The original Williamson amps used the 807 (6 volt version of this tube) and the 6SN7 tubes, so it’s not a new concept.
 
It does sound wonderful.

Yes, totally agree with the use of regulated screens with this tube. I would not have commissioned the amp any other way.
 
Freo-1 said:
It does sound wonderful.

Yes, totally agree with the use of regulated screens with this tube. I would not have commissioned the amp any other way.
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I have an improved Williamson 1625 that will be getting re-visited thanks to this thread. It sounds very good as is, but I bet it could be better (and safer) with the reg. screen option. Thanks for the thread, I'm printing it out to store with the amp. :thmbsp:
 
jaymanaa said:
I have an improved Williamson 1625 that will be getting re-visited thanks to this thread. It sounds very good as is, but I bet it could be better (and safer) with the reg. screen option. Thanks for the thread, I'm printing it out to store with the amp. :thmbsp:
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Excellent! Make sure you post some pics when completed. I'll try to post one or two pics of the underside soon.
 
The amp is breaking in quite nicely. To me, it sounds better than my old Citation II (and that sounded great). Very impressed with the 12SX7/1625 combo. :thmbsp:
 
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