ST70 works with PAS but not another preamp

[lduncan]

I'll try to be quick. I have a Dynaco PAS3 and ST70. Pair appears to work fine but I did once have to wiggle a 7199 input tube to restore sound to a channel. PAS is basically stock. ST70 has caps replaced and some mod for extended bass ?

I recently tried an AES AE-3 preamp into my ST70 and things went haywire. Had very soft sound from one channel and the other channel made my speaker drivers wobble like a subsonic signal. Switched input tubes problem followed. Went to check bias found the quiet side to be a bit high at 1.7 and the noisy side to be very high (5+ if thet can even be accurate).

So I adjusted bias - one side held at 1.5 the other side had to be turned down to about .5 - .6 to eliminate strange speaker wobble (I don't know what else to call it). Sounded fine at that point but I turned it off due to obvious reasons.

The AES preamp works fine into another set of amps. I suspect a bad input tube but I am baffled why it would basically be OK with the PAS but not the other preamp.

Thanks in advance

[DougL]

Does your AE-3 have some DC on the output?

Also sounds like your system is motor-boating (Low frequency feedback)

Filter cap could be bad, and other preamp is putting out something that is exciting it.

[kavermei]

Ground loop perhaps? PAS3 is not grouded IIRC, is the AES preamp grounded?

[lduncan]

Yes the AES has a ground plug if that's what you mean. I believe for whatever reason this has to do with a bad tube. I did swap tubes once more and the problem followed. Stable bias on one and had to crank it down to about .56 on the other to get any usable sound. It actually sounds fine but I don't want to operate that way.

The AES has an odd wiring scheme which apparently requires the speaker connection to be reversed on most amps. I didn't know this (just brought it home for audition). It actually did sound better with my other amps this way.

I am by no means an expert but my suspicion is a bad input tube being magnified by the preamp due to some DC. Which i think is what the above poster was getting at.

[indianajo]

I can't download the pas3 schematic (locks up my computer), but I have traced my PAS2, and the plate of a 12ax7 pulled up by 250000 to B+ goes through a .05 uf paper cap, then through a 250 meg volume control, through some balance stuff appearing to be about 250 meg, and out. This is a lot higher impedance than anything transistorized was designed to be. Early dynaco transistor amps were specially designed to handle the high output impedence of the PAS2 and 3, other transistor amps reportedly had problems. Again, the ST70 is designed for the high input impedence. My ST70 has worked okay with transistor radios as the input device, but I don't know every transistor radio would work as well. I do know I built a phase shift oscillator to balance the preamp channels with - it worked fine on the RIAA input of the PAS2, but drove an op amp mixer into saturation because I left DC on the output between the tips and rings of the rca jacks of the oscillator. The mixer had no input capacitors. So, everything is different.

[Conrad Hoffman]

Wild guess, but I've had problems with a slightly leaky input cap. A cap can measure ok in every respect, but have excess DC leakage. This will foul up the operating point depending on the output characteristics of the preamp. If one preamp has output caps, it might work fine, but if the preamp had a low DC output impedance, the defective amp channel would misbehave. Check those crummy old paper caps.

Conrad

[kevinkr]

Originally Posted by Conrad Hoffman

Wild guess, but I've had problems with a slightly leaky input cap. A cap can measure ok in every respect, but have excess DC leakage. This will foul up the operating point depending on the output characteristics of the preamp. If one preamp has output caps, it might work fine, but if the preamp had a low DC output impedance, the defective amp channel would misbehave. Check those crummy old paper caps.

Conrad

Note that the Dyna ST-70 does not use input caps. Check the AES pre-amp for both DC and subsonic signals at the output.

If you have not replaced the quad cap in ST-70 this should be done soon. One of those 7199 sounds like it is toast.

[lduncan]

Thanks all for your input. I am fairly certain I have a bad input tube. I have no backups due to the amp having the original 7199 board and we know those tubes are hard to find and pricey. All the caps on the ST70 board have been replaced and I think I have found a set of 7199 tubes at a decent price. If not I am going to go ahead and replace the board and have a pair of known good tubes.

I have also decided to go ahead and rehab the PAS preamp. I have listened to it in basically stock form for about 2 months and I think with some work it will be a very pleasing unit for my ears.

LD

[atmasphere]

I suspect a short in the driver tube and it is putting a DC voltage at the input of the amplifier. The DC must be affecting the feedback loop of the other preamp is my guess. A DC voltmeter at the input of the amp would answer that question anyway.