Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mark on February 02, 2019, 09:51:41 pm

Title: Hum in SVT power amp
Post by: Mark on February 02, 2019, 09:51:41 pm: Hi

I have been looking at the power amp of my brother's Japanese made Ampeg SVT for months and I can't locate the reason for it's hum.

The source of the hum is the first 12DW7 stage of the P.I. stage prior to the splitter. If I earth the input to the power amp and the input to the splitter the power amp is acceptably quiet.

I tried making a DC heater supply for the two 12BH7's and 12DW7, which made no difference.

I've tried a different filter cap and different earthing points for the supply and no difference.

Look forward to your thoughts on the SVT.

Regards

Mark

https://ampgarage.com/forum/viewtopic.php?f=6&t=31414&hilit=Ampeg+SVT
Title: Re: Hum in SVT power amp
Post by: Ritchie200 on February 03, 2019, 02:30:34 am: Hi Mark,

Have you seen this?

https://music-electronics-forum.com/showthread.php?t=15670

Jim
Title: Re: Hum in SVT power amp
Post by: Mark on February 06, 2019, 05:30:38 am: Sorry for the late reply Jim, that was interesting to read, but that isn't my brother's amp. His amp was built in the early eighties in Japan. It is very much like the original SVT's.

I dare say there is some sort of ground loop going on in my brother's amp.

Regards

Mark
Title: Re: Hum in SVT power amp
Post by: SoundmasterG on February 06, 2019, 12:23:54 pm: Quote from: Mark on February 06, 2019, 05:30:38 am
Sorry for the late reply Jim, that was interesting to read, but that isn't my brother's amp. His amp was built in the early eighties in Japan. It is very much like the original SVT's.

I dare say there is some sort of ground loop going on in my brother's amp.

Regards

Mark

Sounds like your brother's SVT is an MTI made one. I used to have one of those. The earliest of those used American transformers with the Japanese parts and were basically the same as the American SVT's made by Magnavox. That is what mine was. Then they changed to Japanese transformers and voltages were about 5% lower. Those are still great amps. Then they changed the circuit and went away from the 12DW7's and used 12AX7's. Those aren't as good as the earlier ones. With all of them there were little mistakes sometimes where they overlooked connections and what not. I had a missing jumper wire in mine so all the bass in the channel with the inductor on the tone circuit went through a 0.022uF cap instead of a 0.22uF cap as it should have been when switched and made that channel sound bad for bass.

So i guess what I would say is to look around with a fine tooth comb and make sure the circuit is build as it should be in the schematic. Then after that, look for a ground loop or something that could be causing the hum. Also did you try a different 12DW7? For vintage tubes those seem to be noisy more often than other types.

Greg
Title: Re: Hum in SVT power amp
Post by: Ritchie200 on February 06, 2019, 01:26:50 pm: Hey Mark, sorry about that - I didn't know there were so many versions! I used to re-tube my bass player's SVT back in the 70's and early 80's but it was rock solid. It certainly is an amazing amp. He also had a Model T that I was always working on!

Just for giggles, I would lift the legs of the clamping diodes before the 12DW7 and check them out.

Jim
Title: Re: Hum in SVT power amp
Post by: Mark on February 09, 2019, 09:48:16 pm: I've been told the clamping diodes are there to maintain stability in the amp.
Title: Re: Hum in SVT power amp
Post by: Ritchie200 on February 10, 2019, 11:55:30 am: I'm not saying to get rid of them, just check them out. To do that you will have to lift their legs. Sorry, I should have been more specific.

Jim