Convert Gates pre-amp to DC filament? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 4th October 2011, 06:56 PM   #1
slor is offline slor  United States
diyAudio Member
 
Join Date: Jul 2007
Default Convert Gates pre-amp to DC filament?

I recently rebuilt / restored an old Gates "Dynamote" 2-channel mixer. They're really wonderful-sounding units but a tad noisy. I had hoped to convert this one to a quieter DC filament supply but I'm not sure that's possible with the current setup.

It's my dim understanding that to implement a regulated DC supply, you want the transformer to supply at least 3 "extra" volts to account for losses in rectification, filtering, and regulation. Right now the supply is at 6.6VAC, which is actually pretty good considering the transformer was designed to run on 115V.

Anyway, short of adding a dedicated filament transformer, is there any way to get there from here?

Many thanks in advance!
Attached Images
File Type: jpg Gates PS.jpg (121.1 KB, 143 views)
  Reply With Quote
Old 4th October 2011, 07:27 PM   #2
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
I remember those mixers.
Is the noise 60Hz or 120Hz hum? Is it white noise?
__________________
Frank
  Reply With Quote
Old 4th October 2011, 07:41 PM   #3
slor is offline slor  United States
diyAudio Member
 
Join Date: Jul 2007
Hm, I hadn't actually thought to characterize it other than "tube noise."

By that I mean more hi-freq stuff than AC ripple. All the caps--filtering and coupling--are new. As you know, these units have REALLY high gain and compared with modern preamps, the noise floor is just a bit high. That's why I thought that a DC supply would be a useful mod....
  Reply With Quote
Old 4th October 2011, 07:47 PM   #4
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
A DC supply will not improve the noise figure.
These mixers were designed for remote broadcasting 50 years ago.
Back then, noise wasn't that important. They were generally connected to equalized telephone lines.
You may be able to reduce the gain on the output stage and run the channel and master level controls higher.
Can you post the schematic for the program output amplifier?
__________________
Frank
  Reply With Quote
Old 4th October 2011, 07:54 PM   #5
slor is offline slor  United States
diyAudio Member
 
Join Date: Jul 2007
Here's the closest schematic I have, it's for a unit with remote gain control so the pots (1 for each input, and a master output) aren't shown....
Attached Images
File Type: jpg DYNAMOTE6.jpg (95.1 KB, 126 views)
  Reply With Quote
Old 4th October 2011, 08:20 PM   #6
diyAudio Member
 
trobbins's Avatar
 
Join Date: Feb 2009
Location: Melbourne, Oz
I agree that you need to understand what type of noise is of concern and where it may be being generated.

Noise from some resistor types may be an issue for olde amplifiers.

The impedance of C3 in the humdinger DC elevation may be relatively high, and not bypass heater coupled AC hum currents adequately. Similarly, some more hum reduction may come from increasing C3 in the first amp stage.

Grounding scheme and wiring dress may not have been optimum.

Capacitors were relatively costly and scarce back then - C2 could be significantly increased is size to pull B+ ripple voltage down.

Some techniques for localising noise are using a spectrum analyser of output waveform, along with an AC voltage meter with good low voltage resolution and mains earth loop rejection; battery powering the input valve heater; grounding the grid directly to cathode at the valve base of the first few amp stages.
  Reply With Quote
Old 4th October 2011, 09:57 PM   #7
ChrisA is offline ChrisA  United States
diyAudio Member
 
Join Date: Jan 2008
Quote:
Originally Posted by slor View Post
Hm, I hadn't actually thought to characterize it other than "tube noise."

By that I mean more hi-freq stuff than AC ripple. All the caps--filtering and coupling--are new. As you know, these units have REALLY high gain and compared with modern preamps, the noise floor is just a bit high. That's why I thought that a DC supply would be a useful mod....
No. The DC supply will not help the noise floor unless the problem is low frequency hum. You can prove this by using a 6V battery on the heaters and noting if the noise changes. Only if the 6V battery solves the problem, then go for DC heaters

Looking at the schematic I'd expect a lot of "tube noise" that is just the nature of pentode preamp tubes and this amp has it's share of those for sure.

Last edited by ChrisA; 4th October 2011 at 10:01 PM.
  Reply With Quote
Old 4th October 2011, 11:14 PM   #8
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
With three high gain pentodes in cascade I'm not surprised that you have noise. You might try the old trick that Hewlett-Packard and Boonton Electronics used in their old tube millivolt meters of starving the input tube 6V filaments by running them at 5 or even 4 volts to reduce noise. They would also use selected low noise types like Telefunkens. But that's an expensive option in todays NOS tube market.
__________________
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." ~Thomas Sowell
  Reply With Quote
Old 5th October 2011, 04:02 AM   #9
slor is offline slor  United States
diyAudio Member
 
Join Date: Jul 2007
Quote:
Originally Posted by trobbins View Post
The impedance of C3 in the humdinger DC elevation may be relatively high, and not bypass heater coupled AC hum currents adequately. Similarly, some more hum reduction may come from increasing C3 in the first amp stage.
Interesting, interesting...I'm learning a lot here. I'm not sure I quite understand the comment about C3 in the heater circuit. Can you explain the bit about "not bypass heater coupled AC hum?

As for increasing the value of the OTHER C3, the cathode bypass cap on the first tube, would that also increase gain and bass response?

I'll try the 6V battery test, that's such a simple and obvious strategy....

Thanks much!
  Reply With Quote
Old 5th October 2011, 08:51 AM   #10
diyAudio Member
 
trobbins's Avatar
 
Join Date: Feb 2009
Location: Melbourne, Oz
Hum voltage couples to the grid via stray capacitance - this mechanism is minimised by the humdinger pot, which effectively couples equal and opposite signals to the grid, such that they null.

Hum voltage also completes a current loop via the resistance between heater and cathode, the cathode bias circuit, and the heater/humdinger back to ground. Increasing the heater cathode resistance (by DC elevation) will lower the hum current flowing in this loop. Lowering the impedance of the cathode bias circuit (C3 in amp circuit) will lower the hum voltage developed across the cathode bias circuit. Lowering the impedance of the humdinger wiper to ground (C3 in power supply) bypasses any HT related hum voltage, which would otherwise add itself into the loop.

Hum voltage completes that same loop via capacitance between heater and cathode - and the same comments as before are valid, except that the heater-cathode capacitance doesn't change with DC elevation. It seems that capacitive impedance is much lower than conduction related resistance for the heater-cathode interface.

Yes cathode bypass C3 will change the bass response and increase gain (compared with not having a bypass capacitor), but one of the main advantages of capacitor bypassing the cathode resistance in the first preamp stage is actually the reduction in hum provided.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
DC Blocking on Pre-Amp Puffin Chip Amps 8 22nd January 2009 08:08 PM
to convert a 12v dc car adapter to support 19 volts dc shadiedog87 Power Supplies 6 31st October 2007 07:27 PM
DC filament guitvinny Tubes / Valves 5 16th October 2006 02:15 PM
gates tube pre-amp info?? goingblind Tubes / Valves 0 6th January 2005 12:12 AM
DC in Pre-amp output Herman Chung Tubes / Valves 11 5th August 2004 10:14 PM


New To Site? Need Help?

All times are GMT. The time now is 03:13 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2