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DHT amp - 4p1L driving GM70
DHT amp - 4p1L driving GM70
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Old 28th September 2017, 04:50 PM   #1
vcoustics is offline vcoustics
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Default DHT amp - 4p1L driving GM70

While looking at other power DHT I came across the GM70, great reviews, cheap tube in huge ebay quantities. (short life at 1000 hrs tho).

With a -110 bias @ 1020V B+ it will only run in A1.
The 4p1L at 2.5V input swings 160V pp.
In LTSpice sim at 2.5V input with the schematic attached I achieve 22Watts. Wont need over 20 ever.

Thoughts?

Have read the GM70 "'sound' <-for all that's worth" is between 300B and 211.

Easier to drive then 211?
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Old 28th September 2017, 05:04 PM   #2
kevinkr is offline kevinkr  United States
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DHT amp - 4p1L driving GM70
I've been running GM70 amps since 2011 with 1:1 interstage driven by D3A running at 20mA, with Lundahl 1:1 input transformer. Output transformer 7K primary. 1kV @ 100 - 120mA (copper or graphite plate) Fixed bias about -90V to -100V.

Happy with the result. Tube life is short.
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Old 28th September 2017, 05:15 PM   #3
vcoustics is offline vcoustics
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Thanks kevinkr, tube life is short due to fill req and massive power dissipation probably. I will look at some 7k OPT as well and maybe run the tube at a higher current. Doesn't look like Lundalh has a nice 7k ish OPT, some 6.5k and 9k options only. will keep looking tho.

would you say this tube is better/worse then the 211?
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Old 28th September 2017, 05:23 PM   #4
kevinkr is offline kevinkr  United States
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DHT amp - 4p1L driving GM70
Different, and more linear than the 211 at reasonable plate load resistances. I have not met too many 211 amps I liked.

I heat them using Rod Coleman CCS and average about 1200hrs of life.

I run 100 - 120W of plate dissipation, the coppers definitely should not be run much over 100W and will give slightly longer life.

I am currently running copper plates at 100mA and am up to 1500 hours now, replacement will be needed soon, but they still sound ok at the moment.

My output transformers were made by Electra-Print back in 2010, and were very reasonably priced. They perform very well, they are the only transformers outside of the PSU that have not been replaced at some point with better ones.
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Old 29th September 2017, 01:06 PM   #5
vcoustics is offline vcoustics
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I've searched for hours to find a T that operates in the range I want. Looks like I will go with the Electra-Print option, most options available. Planning on a 8k:8 at 100mA. If I run the tube at Q~ 100mA/1000V and want to build some room for future tinkering would ordering the 120mA version effect sound or stick to the 100mA transformer? Its just a different gap distance but wanted to check.

I had a PM on how I plan to get a 80V swing from the 4p1l and the key is the input transformer 1:4 (bal input)(LL7903) 2.5V:10V, the 4p1l is biased at -12 and yes I would be pushing the tube all out but why not.

Checkout this post by Ale:
4P1L driver – LL7903+LL1671 tests – Bartola Valves
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Old 29th September 2017, 03:33 PM   #6
audiowize is offline audiowize  United States
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Have Sowter make what you need.
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Old 2nd October 2017, 07:10 PM   #7
vcoustics is offline vcoustics
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I have multiple people vouch for the electra-print, I will go with those, great price too. and made in the USA.
Running these at 100Watt dissipation to extend tube life as long as possible. I will update as I get parts and start putting this together.
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Old 2nd October 2017, 07:26 PM   #8
kevinkr is offline kevinkr  United States
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DHT amp - 4p1L driving GM70
I've been very pleased with the output transformers Electra-Print has made for me.

Note that you can run the GM70 at any dissipation within reason where the plates don't glow and tube life is not materially improved or adversely impacted.

The only thing that helps at all is to use CCS filament heating which reduces the inrush when cold and reduces the flapping that will occur at high initial inrush currents.

Running them much beyond 1200 hours is a risky proposition if one of the filament breaks and shorts to the grid or elsewhere - for this reason I replace before they actually fail. Usually the magic goes away right around the time when replacement would be advisable.

I have noticed a marked reduction in emission near the end, perhaps a 15 - 20% drop in plate current since last adjustment is a reliable indicator it is time.
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Old 3rd October 2017, 01:47 PM   #9
vcoustics is offline vcoustics
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Good to know kevinkr! I am excited to begin this. GM70s arrive in a week. Yes I will be using the rod coleman for filaments. I currently use it on my 26 preamp and its solid.

Did you order the 120mA ouput T version? and just run it at 100mA for copper version, 120mA for graphite?

Based on the options electra-print does: 8k, 120mA, 30watt?, 8ohm, copper.
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Old 3rd October 2017, 02:27 PM   #10
kevinkr is offline kevinkr  United States
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DHT amp - 4p1L driving GM70
7K/140mA/40W Copper 8 ohm secondary.
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