Amp Help problems

[mikey5]

Ok I was just playing my amplifier orange TT and all of a sudden the volume went waayy down and then shot up real quick and then went down again. Sort of like a light bulb dimming and then going bright again. I was using my distortion and chorus at the time and then a shut them off and it did it once again, and then normal. So I am thinking that either a power surge drop thing or maybe bad trannys. Fortunately I have yet to switch to switch to my new trannys. Any Ideas guys. Bad tube maybe??

[hamfist]

90% of problems in tube amps are tube-related.  Most likely in your case is power tubes, or a bad tube socket/tube pin connection.
  Try removing the power tubes, and putting them in and out of the sockets a couple of times (if you have some elctrical contact cleaner then spray it onto the tube pins first). If there's some crud or corrosion on the pins or socket that's causing an intermittent connection, then that will probably correct it. If that doesn't work, I'd try some new EL84's.
  If that still doesn't work, then you could try putting a known good 12AX7 in place of each of the pre-amp tubes in turn to see if that cures it (however, it doesn't really sound like a pre-amp tube issue to me).
  If none of that cures it (and I expect it will), it's time to take it to a tech.

Good luck Mike !

[sjwebb90]

Sounds like the EL84's maybe starting to die. I had a similar occurance on my Cornford Hellcat. The volume would drop in and out and then come back. Eventually there was no output. Replacing the EL84 valves fixed the problem.

If you replace your valves you may want to check the biasing on your TT. I had a Made in Korea TT variant where the bias was way to high in the 15W mode (coincidentally there was 15Watts on the valve plate instead on the  nominal 12W) and too lower a bias current in the 7W mode. Too higher bias current will reduce valve life.

When the switch is flicked between the modes, all you are actually doing is just reducing the voltage from around 325V (15W) to around 250V (7W). There is a fixed 120 ohm resistor so the biasing is not really being corrected either way. I found I needed a 160 ohm resistor for the 15W mode. A 68 ohm in parallel with the 160 Ohm in the 7W setting. However no one amps the same, so it would need checking by an amp tech to get it right.

[mikey5]

Thanks so much. In the event that I need to get new tubes what brands do you recommend for the El84 and the 12ax7s? Also about this biasing most techs should be able to do that right??

[jpfamps]

Most amps that use EL84s run them hot. The 12W maximum plate dissipation seems often to be a deemed a minimum rather than a maximum!

In most AC30s the idle dissipation is around 17W per valve, and in the RI Marshall 18W I've measured 19 W per valve!!

The main problem is that the resistor required to bias the amp "correctly" would not be a standard valve. My guess is also that no one resistor value would allow the amp to biased "correctly" in both 7W and 14W modes.

There are mods on the web that switches the resistor value so you can have control over the bias in both modes.

Any tech that can't rebias this amp should not be trying to repair valve amps!

Personally I like JJ EL84s, although they do tend to draw more plate current than other brands. I've had OK results with bog-standard Sovtek valves, but have had less good results with EH. Bare in mind we get through about 100 or so EL84s a year, this is based on a reasonable sample size.

However, I would like to add the caveat that there does seem to be batch variation in valve production, so maybe my experiences with EH valves reflects this.

[hamfist]

In my experience, both JJ and EH EL84's are just fine. The EH's can be a tiny bit brighter than the JJ's, but there's not a whole lot in it.

For 12AX7's, personally I get old tubes for the best tone, but if you're going for modern ones JJ's, EH's and Tung Sol are all OK.

[jpfamps]

The main issue I had with EH EL84s is reliability (we had quite a few fail), and they could sound a bit brittle in the wrong amp. However, as we stopped using them a couple of years ago we could have been victim of a bad batch.

Regarding 12AX7s, I used to really like the EH-12AX7, however we have been advised by our supplier that the current production EH pre-amp valves with a spiral filament are not very reliable (this includes the Sovtek LPS and Tungsol 12AX7). This is a real shame. We are currently using the long plate JJ-ECC803s with good results.

I was not overly excited by the standard JJ-ECC83 and have tried some recent production Chinese 12AX7s, which were quite harsh and expensive (they were allegedly "specially" selected).

I expect our No. 1 priority is that the valve we use are reliable, however we also do need be able to supply good-sounding valves at a reasonable price.

[hamfist]

The main issue I had with EH EL84s is reliability (we had quite a few fail), and they could sound a bit brittle in the wrong amp. However, as we stopped using them a couple of years ago we could have been victim of a bad batch.

I think they've improved a bit since then.  But I definately bow to your experience of way higher numbers of valves than I ever get through. Mine are only ever for personal use, so I go through a very small sample size really.