[fross]

A few years ago I upgraded my Stereo 70 to the AVA Super 70i. Needless to say, I love this amp!

In the past week, the amp started to intermittently blow fuses. The situation is this: The amp will play for hours on end, absolutely clean (no pops, hisses or noise of any sort). But when I turn the unit off, wait a few hours or the next day, the LEFT channel will not function, the 5AR4 tubes go haywire and then the fuse (3 amp slo-blo) blows. It seems that after waiting several more hours the amp will play again continuously, but then after turning it off, returning later and turning it back on, the amp exhibits the same problem again.

I have swapped and re-biased the tubes, changed wires running from the Super PAS 3 to the amp, to no avail. As a final trouble-shooting measure, I have completely disconnected the amplifier from the entire system, to find that the exact problem still exists (only on the LEFT channel).
 

Frank was kind enough to respond to an email, explaining to look into a faulty tube socket or a possible ground issue. It seems that one of the power tubes is in fact difficult to insert/remove from the socket, so this could possibly be the culprit (I really hope so!). However, examining the inside, everything appears to be firmly seated.

Since I retained the original Dynaco sockets, I figure I may as well replace all of them, so went ahead and ordered replacements online. In the meantime can anybody suggest where else to look for the problem? As far as a grounding issue is concerened(?), I'm not even sure where to look, as everything appears to be in order.

[avahifi]

If this was a kit built unit, I suggest resoldering all solder connections.

I also asked if you had checked the bias settings?  Does it set to normal values (1.56V DC)?  I would do this and resolder all connections before replacing tube sockets.

Check the hardward fastening the tube sockets to the chassis.  The power tubes sockets connected to the 15.6V precison resistors much have a good chassis ground connection.

Swap the power tubes from one channel to the other, one at a time, reset bias, and see if the problem changes channels.

Frank Van Alstine

[Listens2tubes]

You say "the 5AR4 tubes go haywire and then the fuse (3 amp slo-blo) blows", why not check the rectifier for shorts. Have you tried reversing the 5AR4's? I had a similar problem with my MkIV amps where the 5AR4 was shorting after about 30 seconds with a flash.

[iclark]

fross, did you ever resolve your problem? The reason I ask is that I have just acquired a 70i. It was working fine, in fact it was wonderful! I decided to remove the bottom cover to look at the workmanship (kit not factory). Every thing looked very good so I put it back together. I did replace the power plug. Now when I turn it on all the tubes light up, but the Rectifier generates a flash after about 30 seconds and the fuse (3amp SB) blows. Time for a new rectifier?


ian

[avahifi]

Your issue is most likely a failed rectifier tube (5AR4) replace it with a new Sovtec 5AR4 tube as we find these are the most durable available right now.

Frank Van Alstine