O/T Harley Benton GA5 VJ Clone preamp tube question

Started by frequencycentral, May 22, 2009, 02:36:43 PM

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frequencycentral

I've had this little amp for a good while now. Nice little amp, and way cheap for what it is. It is apparently made for Thomann in the same factory that Epi Valve Juniors are made in, has the same PCB even.

I've had a couple of issues with it. The PT has three plastic fuse holders, two of which have failed, resulting in the internal spring popping them open, and the amp therefore powering down. The other issue is some sort of sympathetic vibration, which is quite slight and only occurs on some notes, really only very noticable when playing really quietly.

Today I repaired one of the fuse holders and decided to try tracking down this annoying oscillation. I took the sheild off the (unbranded Chinese) 12AX7 and the stay off the (Sovtek) EL84, no joy. Then I pulled the 12AX7 and subbed in a Penta Labs 12AU7. The overall volume of the amp about halfed, which I expected. It sounded pretty much the same toneally though - and the annoying vibration/oscillation was gone. So I'm putting it down to the cheap unbranded Chinese 12AX7. Has anyone experienced similar, and would agree which my prognosis?

I'm just wondering if anyone can recommend a replacement for it? ie which 12AX7 manufacturer would you recommend? TBH I'm pretty lost with 'big' tubes, almost all my tube experience is with subminis!
http://www.frequencycentral.co.uk/

Questo è il fiore del partigiano morto per la libertà!

nelson

Sounds like a cheap microphonic tube, or a loose connection somewhere.

Since it's only one 12AX7 and its individual sound is so key to the amps character, why not go for a NOS mullard or telefunkon or something?

Failng that, for current production I like JJ tubes.

It's personal preference really.


I have the same amp. I haven't noticed any oscillation etc and mine still has the stock tubes.

I am not proud to admit, I have only opened mine up once to see the workmanship.



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Salvatore

Oh cool, a thread about the GA5.
I heard about the problem with the fuse holders, many replaced them already.
Concerning the preamp tube, I would try an other 12ax7, ecc83, whatever.

I have a problem with my own GA5.
At first, after several seconds it works fine for a few seconds, and then fades out for a minute ore 2, and then works again for a few seconds, and fades out, etc.

I replaced the 68k r1 for a 1m, wiggled the 2 tubes a bit, and it worked great,,, for a while.
Now it fades away again...
As far as I can tell, it fails especially when I play at softer volumes.


frequencycentral

Quote from: Salvatore on May 23, 2009, 09:04:34 AM
I have a problem with my own GA5.
At first, after several seconds it works fine for a few seconds, and then fades out for a minute ore 2, and then works again for a few seconds, and fades out, etc.

That's what mine was doing before I 'repaired' the fuse holders. One went a year ago, another this week. My repair consisted of using some wire twisted at each end of the fuse holder, tightened by twisting it with a pair of pliers. This pulled the fuse holder back together. It might be worth soldering the wire twist so it doesn't come undone, but I used very stiff wire.
http://www.frequencycentral.co.uk/

Questo è il fiore del partigiano morto per la libertà!

Salvatore

Great, thank you frequencycentral. :icon_razz:

I fiddled with one of those holders, and it immediately came apart,,
Replaced it with a new one, and all is working like a charm, love it (does a happy dance).

Did I say thanks already ?

(Performs my blistering mystic Tapdance to show gratitude)

Thanks !  :icon_mrgreen:

frequencycentral

Quote from: Salvatore on May 28, 2009, 11:42:43 AM
Great, thank you frequencycentral. :icon_razz:

I fiddled with one of those holders, and it immediately came apart,,
Replaced it with a new one, and all is working like a charm, love it (does a happy dance).

Did I say thanks already ?

(Performs my blistering mystic Tapdance to show gratitude)

Thanks !  :icon_mrgreen:

Case solved! Glad that you got it sorted! I've done two of the three in mine, I guess I should do the third before it pops itself out............major design flaw though.
http://www.frequencycentral.co.uk/

Questo è il fiore del partigiano morto per la libertà!

Ripthorn

One thing you might try Rick is a 5751.  A little lower gain than a 12ax7 (mu of 70 as opposed to 100) but they seem pretty cheap.  Of course, a 12ax7 might run the same as a 5751.  But it's always something to try to see what happens.  Rumor has it SRV used the 5751's a lot.
Exact science is not an exact science - Nikola Tesla in The Prestige
https://scientificguitarist.wixsite.com/home

frequencycentral

Sorry for the necrobump, but I just had to mention that I just replaced the stock Chinese 12AX7 with a vintage Brimar 12AX7 with the square getters. My 'annoying oscillation' has completely gone and the vintage Brimar has instantly transported me back to a time when Elvis was king and Brylcreem was his queen. I was looking for a Mullard, but got a real good deal on the Brimar on the 'bay.
http://www.frequencycentral.co.uk/

Questo è il fiore del partigiano morto per la libertà!

petemoore

http://en.wikipedia.org/wiki/12AT7
  and any other letter you can find that goes in the 12a_7 missing position.
  plug in replacement 12a_7's offer various gain Mu, there's a chart somewhere, IIRC the 12ax7 is 100mu, t7 is 60 mu, the 12au7 is 40mu. Notice the 'Mu' and 'current' ratings on data sheets.
  Thetubestore.com has some 'pro-blurbs' about tube construction, there are other tube sellers and tube reviews online, one such site offered field-test results, though 'one tube set tests' are suspicious, reading all of that gave me some ideas of what to expect from this and that tube brands.
  Lower gain in the pre or PI tube sockets = reduction of ''hard-gainey'' sound. I find I can replace that gain using active and 9v, very nice 'n easy.
  I have the theory these amps are setup for high gain so you can plug an unboosted, not so hot pickup in and make it do output tube distortion.
  All these pre and output tubes that look different, sound different.
  I tend to go for the 'premium' and 'long life' types, never spending more than about double what the bottom price tube would be, preferring greatly when I found the old RCA Blackplates, very nice, IMO are just worth way too much dollar wise, I'm not sure what I should be hearing over the 'premiums'.
  ''Premium'' fuse holder...highly recommended.
Convention creates following, following creates convention.

Renegadrian

Glad to read that Rick! Sometimes swinging tubes gives you so many different results!!!
Done an' workin'=Too many to mention - Tube addict!

frequencycentral

Quote from: Renegadrian on August 06, 2009, 10:11:04 AM
Glad to read that Rick! Sometimes swinging tubes gives you so many different results!!!

Yeah but now I have a dodgy microphonic Chinese 12AX7 that I'll be just itching to do something with..................
http://www.frequencycentral.co.uk/

Questo è il fiore del partigiano morto per la libertà!