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DHT amp - 4p1L driving GM70
DHT amp - 4p1L driving GM70
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Old 3rd October 2017, 07:47 PM   #11
hooman is offline hooman  Iran
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ML3 is a single-ended and lamm use circuit like srpp for drive gm70. sound of amps like this is amazing! why use expensive interstage transformer ?sound of 2 interstage transformer !!
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Old 4th October 2017, 03:00 PM   #12
deafbykhorns is offline deafbykhorns  United States
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Default IT Coupled

Check with Monolith Magnetics.
I started out with RC coupling, changed to LC coupling and finally settled with IT coupled. I had a Hammond 1629SE which was really nice, had good bass but didn't have the detail so I ordered a custom Monlith Magnetics OPT. I could of lived with the Hammond but I just wanted something higher end. The Hammond's tend to rolloff which shows up in the modeling I did. The Monolith is an S-5 gapped for 100ma, Inductance was perfect for the GM-70, bass is not an issue. You could go with an S-15 if your running a lower voltage (900-1000) which I think would give you the best results if you don't care about going into the A2 area very much.
If you have LTSpice I could send you some schematics I experimented with.
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Old 7th February 2018, 05:46 PM   #13
vcoustics is offline vcoustics
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Filament Bias vs Fixed Grid Bias with its own PSU ~-100V for GM70?
That is the question.

Originally I was going to do a 5th PSU for the bias but now I am not sure.
(1-HT 4p1l, 2-HT GM70, 3-B+ 4p1l, 4-B+ GM70)
HT psu is going to a Rod Coleman Reg.

In his install documents he even has a schematic of a fulfillment bias for the GM-70, convenient (might have to adjust to proper bias V of course) image attached.
(auto vs Fixed for this size tube?)

Thoughts?
Attached Images
File Type: png GM-70 Rod Coleman.png (138.6 KB, 208 views)

Last edited by vcoustics; 7th February 2018 at 06:08 PM. Reason: Clerify the type of bias better
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Old 7th February 2018, 06:07 PM   #14
kevinkr is offline kevinkr  United States
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DHT amp - 4p1L driving GM70
I'm running GM70 at 1kV with -90V to -95V of bias. Filament bias would result in somewhere around 270W of dissipation in the filament series resistor, plus last time I checked the Coleman regulators could not operate at 115V output.

Filament bias is a total non-starter IMO.

What is shown is what is commonly known as cathode bias which is not the same as filament bias as shown in conjunction with 4P1L, etc..

I am using Coleman regulators in my GM70 amps which have been in continuous use for about 7 years now. They need plenty of heat sink.
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Old 7th February 2018, 06:14 PM   #15
vcoustics is offline vcoustics
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I totally meant to say cathode bias but have been playing so much with the 26/4p1L filament bias that I instinctively say it now without thinking. Of course never filament unless I wanna heat my house and burn Coleman regulators.

Have you ever considered a ~-90v to the secondary of the Interstage Transformer?

Are you using Fixed or Auto?

Last edited by vcoustics; 7th February 2018 at 06:20 PM. Reason: More content
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Old 14th February 2018, 01:58 PM   #16
vcoustics is offline vcoustics
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I have been planning the B+ PSU for the GM-70 and its not as easy as other amps Ive built. Planning on using SS diodes (tubes later years) for rectification. anyone have any recommendation?
All the per-delivered ones in PSUD2 are hitting error PIV exceeded.

I plan on using the Hammond 724 (750-0-750) and Hammond 193S/Q chokes, they are rated max 1kv which is max and not sure if its ok to go a bit higher.
Higher value chokes?
Preliminary PSU image attached.

HT Rod Coleman for all tubes and their PSU were easy part.
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File Type: jpg B+_PSU_v1.jpg (166.7 KB, 136 views)
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Old 14th February 2018, 04:06 PM   #17
kevinkr is offline kevinkr  United States
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DHT amp - 4p1L driving GM70
I used 1.6kVpiv SIC diodes in mine, in a bridge configuration with a 710V primary, more than seven years so far without issues. (They were however tested for reverse breakdown to 2kV at 65C.)

If as I suspect you are planning on using FWCT you can place diodes in series. with appropriately rated resistors and capacitors. Note also that DigiKey last time I checked still had a pretty good selection of high PIV diodes and you should look at what they have available.

I used a CLCLC filter and placed the chokes in the negative rail where they only have to stand off a few volts typically. This trick was often employed in ham gear operating at high voltages to reduce demands on choke insulation.

Don't exceed the insulation voltage rating.

I used fixed bias in the output stage and the interstage transformer is connected to the bias supply which is adjustable. I used a fair amount of film capacitance to decouple the transformer from the bias supply, the resulting time constant means I have to be careful about applying B+ before the bias is fully there. An unregulated floating bias supply using a high voltage n channel mosfet with adjustable gate voltage in the + side of the supply would be the way I would go today to provide a short time constant, low Z bias point to the transformer. LTSpice can help you figure out how to do it if it is not immediately clear. (The problem is p channel mosfets come only in drain voltage ratings of a couple of hundred V and IMO that is just not enough)
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