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"Dun Mei Audio" OTL Amp - anyone seen one before?

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Posted on September 11, 2020 at 07:43:38
mmcc73
Audiophile

Posts: 21
Location: Northeast Ohio
Joined: October 27, 2018
Hi all - I bought this recently in a local online auction. I have not yet picked it up so I don't yet know if there is anything in the chassis or not - I'm really hoping there is. At this point I'm assuming it is an OTL amp because of the OTL sticker on the front.

I'm curious if anyone has ever seen an amp like this. My googling has turned up very little. The chassis is available empty on Aliexpress, and I have not been able to find any pictures of one being used for an OTL amplifier.







 

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RE: "Dun Mei Audio" OTL Amp - anyone seen one before?, posted on September 11, 2020 at 12:07:37
6bq5
Audiophile

Posts: 3405
Location: SF Bay
Joined: August 16, 2001
interesting looking integrated-
will look forward to pictures from under the hood-
Happy Listening

 

If you need help figuring it out, shouldn't be too hard., posted on September 11, 2020 at 13:50:39
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
BTW This amp does not make a lot of power; a 16 ohm loudspeaker is recommended.

 

Dun Mei..., posted on September 11, 2020 at 14:13:40
mmcc73
Audiophile

Posts: 21
Location: Northeast Ohio
Joined: October 27, 2018
I will most definitely need help. I've messed around with tube stuff a little but, but not much.

As you can see in the photos below, the wiring in the chassis looks pretty well done. There are more components under the plastic cover presumably associated with the power supply, but I didn't get photo (I certainly can if it would be helpful).

However, I don't know what the heck is going on under the transformer cover. A bunch of loose wires, bad looking soldering, a couple "6.0A GLASS PASSIVATED BRIDGE RECTIFIER"s (based on the part numbers).

So...
- Any idea what kind of tubes I would need to make this thing go?
- You mentioned low power - how low? 1W? 10W?
- What the heck should I do with that transformer?










 

The amp uses 6AS7G power tubes., posted on September 11, 2020 at 15:28:13
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
They are wired in a totem pole configuration.

The 120V winding is likely the output section high voltage. Is there continuity between the right pin marked '120V' and the 3rd 2nd pin from that (also marked '120v')? If yes then they are the high voltage and the amp is using a bipolar power supply.

Are there other windings on the other side of the transformer? Does it make voltages when power is connected to the primary? If this is intimidating, take it to a technician to see. Not worth getting zapped...

 

Dun Mei..., posted on September 12, 2020 at 09:04:46
mmcc73
Audiophile

Posts: 21
Location: Northeast Ohio
Joined: October 27, 2018
Thanks for the info and suggestions!

Below are three more images - clearer pictures of the front and back of the transformer and a picture of the circuitry directly below the transformer.

I tested continuity on the primary and various secondaries and it was as expected based on the labeling - had continuity between the 110V and 0V on the primary, and in the expected places on the secondaries both front and back.

Some questions...

To test the voltages, I need some sort of load correct? Would something like a 50 watt incandescent bulb do the trick?

I assume the smaller tubes are rectifiers? Do you know what designation would fit the bill?













 

You don't need a load on the transformer, and the smaller tubes are signal tubes nt, posted on September 14, 2020 at 08:21:32
Ralph
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Posts: 3960
Location: Minnesota
Joined: April 24, 2002
-

 

Dun Mei..., posted on September 15, 2020 at 06:59:22
mmcc73
Audiophile

Posts: 21
Location: Northeast Ohio
Joined: October 27, 2018
The fact that the 750mA fuse blew when I applied power implies that the transformer is bad I take it.

Can you recommend a replacement? It is unclear to me if the amp circuit "needs" all of the voltage options the current transformer offers.

 

bad transformer, posted on September 15, 2020 at 08:17:32
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
Well the fact that the leads were disconnected made it a good bet that someone had already sorted out that the transformer had failed.

This is clearly a custom part. So you could have one designed and made up, which would likely run you about $500-$750.00. Or you could use a number of separate transformers to make up the voltages needed. The 6.3V filament winding is probably available as an off-the-shelf toroid. You might be able to find the other voltages except for the B+ for the output tubes- my guess is a custom transformer will be needed for that. Getting all this to fit under that power transformer cover might be a trick. But you might also have room for the smaller transformers (Bias and driver power supply) beneath the chassis.

 

Dun Mei..., posted on September 15, 2020 at 12:03:52
mmcc73
Audiophile

Posts: 21
Location: Northeast Ohio
Joined: October 27, 2018
Thanks Ralph.

The current transformer has the following secondaries. Are you able to determine what voltages are used for what purpose?

V1 - 6.3V - filament
V2 - 25V - Bias?
V3 - 60V ?
V4 - -120-0-120V ?
V5 - -260-0-260V ?

Clark

I had posted this before and deleted it because the subject line was messed up. Unfortunately, right when I did that, Ralph had replied and that got deleted as well. Fortunately, I got his message in email:

V1 - 6.3V - filament high current, about 25 amps to support all the tubes
V2 - 25V - Bias?
V3 - 60V ?
These two are probably bias supplies, one for the top tubes and one for the
bottom?

V4 - -120-0-120V ? Definitely output section, you'll need about 2 amps
V5 - -260-0-260V ? Driver section, probably 50mA will do the job.

 

transformer windings, posted on September 15, 2020 at 12:05:50
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
V1 - 6.3V - filament high current, about 25 amps to support all the tubes
V2 - 25V - Bias?
V3 - 60V ?
These two are probably bias supplies, one for the top tubes and one for the bottom?

V4 - -120-0-120V ? Definitely output section, you'll need about 2 amps
V5 - -260-0-260V ? Driver section, probably 50mA will do the job.

 

Transformers..., posted on September 15, 2020 at 12:16:28
mmcc73
Audiophile

Posts: 21
Location: Northeast Ohio
Joined: October 27, 2018
Thanks once again. How exact do the voltages need to be? Could I get away with, say 250-0-250 in place of 260? How about the bias voltages?

 

RE: Transformers..., posted on September 15, 2020 at 12:26:51
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
250-0-250 would work fine.

The bias probably has to be pretty exact.

 

Dun Mei..., posted on September 15, 2020 at 14:11:39
mmcc73
Audiophile

Posts: 21
Location: Northeast Ohio
Joined: October 27, 2018
Great - I sent an email to the folks at Antique Electronics to see if they had any suggestions for a transformer combo that might fit the bill.

 

"Dun Mei Audio" is not the manufacturer, posted on December 30, 2020 at 22:46:58
bequerel
Audiophile

Posts: 237
Location: NORWAY
Joined: September 29, 2002
Dun Mei Audio is not the manufacturer of this amp. Quite sure. Dun Mei Audio is a big supplier of chassis and hardware for the DIY commnity. The chassis in the pictures (AV-38) is available from several sources. If you look closely, you see that the "OTL" designation on the front has been added by the builder.

 

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