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Old 2nd May 2012, 01:39 AM   #1
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Default 845 vs GM70

I picked up a Mr. Liang 845 SET amp in "FOR PARTS OR REPAIR" condition. It's has several no-big-deal issues and at least one "big-deal" issue. One of the floating filament supplies to the 845 triode is conducting to ground through what seems to be a failure within the circuit card. This failure extends to the 300B filament circuit and blew up some regulators too.

This amp came to me with the 300Bs but with no 845s. Given that I've got to get pretty creative to fix this thing, knowing that a good bit of the power supply card is shot, I can't see why I must stay with the 845s. Since I must replace the filament supply, and there isn't that much spare room under the hood, an external PS may be the best solution and I can build a 20 volt supply as easy as a 10. (I will note here that the other working supply is putting up 12.5 volts so it's not like I don't have to deal with that too)

I'm not going to shell out a couple of grand for a pair of NOS RCA 845s (Not that there's any thing wrong with that) and I'm not really that crazy about paying big bucks for the Chinese 845 given the mixed reviews either.

So I'm interested in some input on the quality of the Russian GM70s. Are they consistent? Are they robust? Has anyone here compared their sound to the 845?

Dave

Last edited by Captn Dave; 2nd May 2012 at 02:00 AM.
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Old 2nd May 2012, 02:16 AM   #2
kevinkr is offline kevinkr  United States
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I've not compared the 845 and GM70 directly, but am familiar with both. I'm running GM70s in my system and am very pleased with the performance - IMHO (YMMV) I've never heard such effortless performance from another dht.. (something about fairly low rp and lots of current, not to mention heat.. )

I've run graphite plates but am currently running copper plate GM70 - I've had no reliability problems with any of the tubes I have on hand, and have heard no talk of problems with this type at all.
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Old 2nd May 2012, 02:56 AM   #3
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Originally Posted by kevinkr View Post
not to mention heat.. )
Yes, let's not mention the heat. I live in Phoenix and I'm aware the GM70 requires twice the filament power of the 845. I may have to set it outside in the summer

Kevin, Didn't I see that you had a Power One power supply in the swap meet back in January? Do you still have that?

Last edited by Captn Dave; 2nd May 2012 at 03:14 AM.
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Old 2nd May 2012, 02:59 AM   #4
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I don't know if there is any way one of tomchr's switching filament supplies could work here. I think the device he chose was a 3A device. Maybe there is a device that could eke out a bit more? Probably worth considering on a monster like this. It might cool things down a bit.
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Old 2nd May 2012, 03:16 AM   #5
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BTW, here is a link to the Mr. Liang site in case anyone is wondering what I'm going on about. 845 Amp


In the second paragraph I'm going to complain about the filament PS design, so let me say in the first paragraph that these amps are really pretty impressive in most respects. They seem to have very substantial iron, the chassis is very robust and the circuit cards, despite a failure (a cap blew a hole in it) are of very high quality. The 300B filament supply is regulated and quite satisfactory.

My big complaint is that the 845 filament supply is inadequate. The filament voltage (under load) is 12.5 rather than 10. The design is bridge rectification followed by 4 x 10,000uf caps. I have seen photos of a circuit card with a couple of additional heat sinks that make me wonder if I don't have a transitional unit assembled from a transformer wound for a regulated power supply but with the old unregulated power supply board. And yes, it is a little more hum than one would like.

I'd love to see the circuit design of a suitable regulated supply. I suppose it would need to be a constant current circuit to stand up to the inrush from the cold filament.

Last edited by Captn Dave; 2nd May 2012 at 03:22 AM.
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Old 2nd May 2012, 03:37 AM   #6
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tomchr's is a switching regulator (constant voltage) but with a soft start. You can bring the filament up as slow as you want.

However, he definitely did not have a tube like this in mind when he designed it. It may not handle the voltage or current. It's worth looking into, though.
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Old 2nd May 2012, 04:38 AM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Captn Dave View Post

Kevin, Didn't I see that you had a Power One power supply in the swap meet back in January? Do you still have that?
As I commented tonight in swap meet I may yet have those supplies - I'll check in the morning as I head out to work and let you know. Hopefully you are local as I had hoped to hand them directly to a new owner.

I'm pretty familiar with the Liang 845 and there do seem to be several versions - they aren't very common here in the U.S.
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Old 2nd May 2012, 08:04 AM   #8
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Quote:
Originally Posted by Captn Dave View Post

I'd love to see the circuit design of a suitable regulated supply. I suppose it would need to be a constant current circuit to stand up to the inrush from the cold filament.
Dave, here's a GM-70 filament regulator already in service in dozens of GM-70 amps around the world. Similar versions are supplying 813s [5A] and even 75TLs [6.25A].

Yes, it is current-driven - this also prevents filament breakage from turn-ON surge for susceptible filaments (Svetlanas, and many modern tubes).

It is designed to get the best possible sound from filament heating, & plenty of DIYers like it:

New DHT heater

A self-assembly kit - PCB + parts, please send email/PM if you'd like the PDF app notes or more details.
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Old 2nd May 2012, 04:21 PM   #9
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Originally Posted by Rod Coleman View Post
Dave, here's a GM-70 filament regulator already in service in dozens of GM-70 amps around the world. Similar versions are supplying 813s [5A] and even 75TLs [6.25A].

Yes, it is current-driven - this also prevents filament breakage from turn-ON surge for susceptible filaments (Svetlanas, and many modern tubes).

It is designed to get the best possible sound from filament heating, & plenty of DIYers like it:

New DHT heater

A self-assembly kit - PCB + parts, please send email/PM if you'd like the PDF app notes or more details.
Thanks Rod. PM sent. I have been reading through that thread but at 450+ post's it's difficult to get to the meat of the matter.

Last edited by Captn Dave; 2nd May 2012 at 04:24 PM.
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Old 2nd May 2012, 04:30 PM   #10
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Hopefully you are local as I had hoped to hand them directly to a new owner.
Well I'm not. But there is an electronics surplus store here in town with a big pallet of Power One power supplies, and I'll probably need two of them anyway, so no worries.
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