I have a couple different 1970s era transistor amps, and I've recently added a Dynaco SCA-35 tube amp. I've been switching back and forth between the two transistor amps for some time, and to make it easier, I ran my turntable ground wire to both screws. I've never had a problem so far. Now that I have the tube amp in place, I was pondering adding that to my "loop" of ground wires. But before I do, I want to make sure this is not a dumb idea on my part. Is this a situation where if a plug was turned the wrong way, I could have one chassis that's "hot" and one chassis that's "neutral" and cause damage? I don't normally have two amps on at the same time. Thanks!

The tube amp is transformer isolated isn't it? (You shouldn't use it if it isn't!) If any amp has a grounded cord, they all will be.

Try it and see. Hum may be a problem.

Thanks for the courage to try it. You are right, the hum increases slightly, but that's about it.

I'd use a heavier gauge wire for my ground loop. (pun intended) .... I use #14 or #12 stranded electrical wire .... the more the better, but at a certain point bigger won't make any difference. You want the lowest resistance and impedance connection possible for your ground buss. (notice I didn't say "loop" again).

If it hums this way, try extending the ground buss all the way to your house ground at the electrical panel. I used a length of #10 bare wire for this "ground extension" here.

Note: This probably does not apply in your situation, but in the case where the separate items are some distance apart, looping them together is not the "best" way to go. In that case, run a separate ground from each piece of equipment directly to a single point, in this case probably the electrical panel ground. This is known as a "star grounding scheme" .... all items ground at a single point. Again, probably overkill for a home setup, but still the best way to go. It's more used in studios and industrial audio/video installations.

Overkill? perhaps... but I had the wire sitting around anyway.....

Thanks, I didn't know wire size could make a difference.

Since connecting them all together caused more hum, I came up with an "interesting" solution to this problem. I got an electrical box and 3-way (aka double pole) light switch from the hardware store. So now the light switch can alternate back and forth between the ground connections for the transistor amps and tube amp. That way they don't interfere with each other.

Hope you don't mind a couple last questions about this Dynaco SCA-35 amp.

I recapped the electrolytics, and the mylar type capacitors in it. It make all sorts of crazy screeching noises through the speakers when turned on. I finally gave up and took it to a local repair shop. The guy was secretive about what he did to it (protecting his company secrets I guess) but a look at the returned parts shows he replaced a lot of the ceramic disc capacitors. As it is, the amp works very well and he did a thorough job of cleaning everything up. But when the amp is warming up, I hear about 4 seconds of a low "moan" type of sound in one channel. When the tubes warm up and start functioning, everything is fine. The sound doesn't bother me, but I just want to ask if this could be hurting anything.

Second question. The amp has three phono inputs- "low" "high" and "ceramic". I'm using a newer cartridge (Audio Technica AT440mla) that has an output around 4mV. Using the "low input" setting sounds great, but sometimes when I'm playing a louder-mastered 1980s era record, I'll hear a brief popping noise when a loud instrument comes in, as if the input is being overdriven. This also happens when I touch the stylus and generate too great of a signal. When I use the "high input" I don't have this problem but I need to turn the volume knob to 75% to barely get normal listening volume. Any advice on this? Perhaps the solution might be to abandon the preamp and purchase a separate tube preamp. I read about how using 6GH8A tubes with converters instead of 7199s (which I am using) can cause instability and popping, but it feels like what is going on here is things being overdriven.

I use the 6GH8 and adapters in all my Dynaco tube amps. No issues with any instability and they sound great.

The PEC that the phono input feeds thru to get to the first section could go bad. They certainly are getting old but there is no substitute. You could try to test between legs after looking at the schematic. Something may show up poorly. Not much you can do about it unless you have a parts amp to pick from.

I bought a solid state professional grade phono preamp. Tube vs solid state here is not going to have any effect on the sonic quality or the "warmness" or how the rest of the system sounds... you want the absolutely best and most exact representation of the signal coming off the record through the cartridge..... and a tube is not better than transistors here, if the external preamp is of the pro grade. Forget the consumer stuff or the old radio shack bricks. Those are noisy and probably not adequately equalized.

It sounds like the gain in the "hi" setting on your preamp might not be high enough... could be any number of reasons for that. I think 4mv output from the AT cartridge qualifies as "hi" but what do I know.