Gates Dynamote hum

[Dmichel123]

I have a Dynamote on the bench that is pretty noisy. It's this model: http://fixthatmix.com/Gates_mixer.php

I have rebuilt the power supply and replaced every capacitor as well as any resistors that have drifted more than 15% or so. The hum goes away with the input 5879 removed, goes down quite a bit with the grid grounded. This thing had been severely modified, particularly the input passive mixer, and I did my best to wire it how I thought it should be. I seem to remember reading somewhere that these were 30 or 50 ohm input, but this had 150/300 ohm Davens in it. I wired input jacks to attenuators with 2 conductor shielded cable: pin 2 to IN, pin 3 to C, and all pin 1s together and to the input end of the ground bus. I've tried grounding the C terminals with zero change in noise. I've swapped several 5879s with not much change in noise.

 

[Dmichel123]

I just noticed this one has a 25uf bypass cap on the second stage that isn't on that guy's schematic. Also the 910k feedback resistor is a 560k in this one. Sure looks original. I guess that schematic probably says 710k, there is a 150k in series with that 560k.

 

[emrr]

I have yet to meet an early Gates remote that didn’t have a good amount of hum from power transformer proximity to input stage/transformer.  Is it the input transformer type that can be rotated? 

Daven; if it’s like many, you could get 50 or 150 input models.  If it were mine I’d lose the mixer and make it a single channel, get that bit of S/N back. 

I’m not against changing the first stage to triode, thereby reducing overall gain and increasing front end headroom.  Could make the noise level at least feel better, even if it isn’t reduced any more than by the gain change amount.  Or wrap that V2 NFB to include V1. 

I might guess that 2nd stage bypass was added, but not sure. 

 

[Dmichel123]

Yea, Doug. If it wasn't being sold, I'd use it as a single channel without the passive mix section up front. I'm gonna try rotating the input transformer and deleting that 2nd stage bypass cap and see what I get. I like triode-strapped pentodes up front, too. Most of my favorite sounding pres from the '40s/early '50s are done that way and work well for me. We're selling this as restored, so I want to at least attempt to get the original circuit working well before doing any modifications. 

BTW, we picked up a handmade console from 1961 based on the Gates SA series amps that is incredible! They used original Gates UTC iron and bent up chassis identical to the original SA-70s, as well as the program and monitor amps (SA-94, SA-134? I don't remember exactly which they are) Exact Gates circuits with the UTCs!! They sound great!! We've recapped them and wired 'em up. Still need to replace some carbon comp plate resistors and get some hiss out. I will try to remember to take some pics to show you!

Here's a drawing of what I did on the input mixer. I tried grounding pin 3 and making it unbalanced and it didn't seem to make any difference. 

 

[emrr]

Cool, love to see those pics. 

That input drawing looks like what I'm used to seeing, usually it is grounded on the C, and I've found the same, it sometimes seems to make no measurable difference if floating. 

 

[Dmichel123]

Here's a quick pic.

 

[emrr]

I wouldn't suspect home made from that pic!

 

[Dmichel123]

I wouldn't suspect home made from that pic!

That's the story we were given, and the chassis labels are stamped into the aluminum rather than the Gates stencils you would normally see.

Back on topic, I lifted the heater CT and installed a hum balance pot. I can get rid of 60Hz hum now, but still have a lot of hiss and a little residual 120Hz...

 

[emrr]

That's the story we were given, and the chassis labels are stamped into the aluminum rather than the Gates stencils you would normally see.

Back on topic, I lifted the heater CT and installed a hum balance pot. I can get rid of 60Hz hum now, but still have a lot of hiss and a little residual 120Hz...

Cool, love to see more if ya can. 

1st stage plate and cathode R's still best bet for overall hiss after tube burn-in/selection.  Good luck. 

 

[emrr]

Ok, off topic on the SA.  The preamps have two inputs on them, the correct AI-3001 input and then the AI-3031 in the output position, which I don't have info on.  Apparently used backwards as an output.  I've seen one other higher ratio AO- output on a very early SA-40.  . 

 

[pucho812]

every time I read this I think of this song because the name is similar.  sorry, I know this doesn't add to the conversation or help any but  little levity now and then is good.


https://www.youtube.com/watch?v=u4hzVpYAP-U

 

[Fuzz Face]

every time I read this I think of this song because the name is similar.  sorry, I know this doesn't add to the conversation or help any but  little levity now and then is good.


https://www.youtube.com/watch?v=u4hzVpYAP-U

My very first thought when I read this! I had to refrain from posting haha

 

[RSRecords]

every get this sorted out? I've got one my bench doing the same thing. It's been fully recapped. 1st 5879 seems microphonic but swapping doesn't change anything. I suspect its just all that gain?

 

[RSRecords]

Traced the 6x4 supply. Does this PSU seem right to you? R1/R2/C3 seem off to me compared to typical schematics around.