Acoustat Answer Man is here

Member
Joined 2016
Paid Member
A BIG Thank you to AcoustatAnswerMan and to all the rest of you who have posted helpful replies in this thread. Last week I successfuly rebuilt the 5kv bias supply in our Acoustat Model 3s.
This rebuild solved a low-output problem with one speaker and both now sound much, much better. Neither my wife nor I wanted to replace these speakers, and now we don't need to do so. Maybe they will last another 40 years. . .
 
Medallion secondary open.

Andy or anyone,
I just had one of my 1+1's go down in the bass. I traced it to the 1 ohm resistor, R4, feeding the LF transformer. I replaced it and it didn't last 3 minutes and things got bad fast. For the first minute or more, the sound was completely as it should be, so I think the panels are fine.
As a note the day I bought them from Dave King, we had the same strange rattling sound like a loose diaphragm. but it went away, returned, went away again. It has been a few months without the death rattle, and now nothing.
So the secondary yellow to white is open, blue to white is about 1.5k ohms.
The two 50k's are fine.
So I believe a new Medallion is in order. I have the blue with the C mod.
Is there a source for this, a reputable company, or ?????
 
It does sound like you may have an open secondary on the LF transformer, but it puzzles me that this would cause the 1-ohm resistor to get hot. Before doing anything else, I would ensure that your amplifier does not have any excessive DC offset in that channel.

As far as replacing the transformer, try contacting Roy Esposito, referenced in an earlier post.

BTW, the "rattling" sound you mentioned is caused by loose Mylar, or possibly foreign material in the gap. It's possible, if you've lost one phase of the LF output, that this could cause a lack of control over the diaphragm, which might increase the chance of rattling.

When you say "go down in the bass" do you mean that bass frequencies suddenly decreased in volume?
 
Last edited:
Thanks for the response. I was referring to going from clean sound to a rattle with significant decrease in volume to complete lack of bass within a 1 minute time span.
I am running that amp on a pair of 2+2 right now with no odd effects so it doesn't appear to be an amp fault. After the failure I opened it up and unsoldered the primaries and secondary and found the open secondary.
Background: R4 was replaced a few years ago with a thick film 100w resistor (T-03P package) and ran ok, except for the peculiar sound Dave and I heard. Three weeks ago I started up the system and no bass, I found the resistor failure and replaced it with a 140w thick film resistor. The result was the one minute sequence above and a hot resistor. My conclusion was a secondary fault hanging on for years that was finally pushed over the edge.
 
I responded to the person who posted the ad above, but then the ad was turned off a couple days later. I am trying to get a replacement bias transformer for one of the 1+1 interfaces on this pair of speakers I just got. Before firing them up, I opened both interfaces and saw one of the bias transformers has evidence of arcing damage - two wires were pressed up against it. I pulled them away, the insulation seems intact, but there is an obvious burnt spot. If anyone knows where I could purchase one of these, I'd be thrilled. They both power up and sound clean, but would like to be able to leave them on all the time with no worries, and get them sounding as good as possible. Thanks for any leads
 

Attachments

  • IMG_20210928_121650935.jpg
    IMG_20210928_121650935.jpg
    578.1 KB · Views: 319
Hi...thats not the bias trans....that the bass trans ....i cant see a bias trans on your pic, but if you can play the speakers an sound good the bad spot my just be on the out side of your bass trans..... in this pic the smaller trans up front brown paper is bias,you my also ck besure about the 1ohm 30watt res. is still 1ohm if not replace it........good luck
 

Attachments

  • DSCF0934.JPG
    DSCF0934.JPG
    739.5 KB · Views: 314
Last edited:
Thanks for the reply, tyu. I should have looked at the wire colors closer. I'd assumed the two smaller ones were the medallions, whoops. I will check that resistor when I open it back up. Would it be beneficial to put some HV silicone on the exposed damaged spot to prevent further arcing? The main fuse on the outside was blown when I got them, replaced that and they're making music. I'm not sure what they've been thru.
 
I responded to the person who posted the ad above, but then the ad was turned off a couple days later. I am trying to get a replacement bias transformer for one of the 1+1 interfaces on this pair of speakers I just got. Before firing them up, I opened both interfaces and saw one of the bias transformers has evidence of arcing damage - two wires were pressed up against it. I pulled them away, the insulation seems intact, but there is an obvious burnt spot. If anyone knows where I could purchase one of these, I'd be thrilled. They both power up and sound clean, but would like to be able to leave them on all the time with no worries, and get them sounding as good as possible. Thanks for any leads

That black spot on the transformer typically indicates internal arcing. It doesn't absolutely indicate a problem, but there's a good chance you need to replace the LF transformer.

The speaker may still play OK at lower levels if you have an internal high-voltage breakdown in the transformer. But at higher levels, the insulation may break down and arcing will resume. Of course this is a problem that will always get worse over time, not better. Putting silicon on the burnt spot won't hurt, but it may not help, either.

Unfortunately this is a common symptom on the LF transformers when driven too hard.
 
Last edited:
Thanks for responding AAM. I've purchased some Super Corona Dope and applied it to the burned area of the LF transformer, but the speaker is cutting out after about ten minutes, the music so faint it is near silent. I tested the bias with the speaker not connected with a non-HV meter and measure 70v at the red pin, so that seems in the ballpark. I guess I'm looking at getting the LF rewound, I really want to get these going. Is there a xfrmr rewind service that you know of who can do these? Or does anyone out there have an extra one of these to sell to me, or even a lead on one? I was hoping the corona dope would do the trick. Are there other things I should check? The other speaker sounds awesome.
 
The transformers from any of the MK121 series of interfaces will work. However, due to product evolution, they may not sound exactly the same. The so-called Medallion transformers introduced in the late 80s were a distinct improvement over the originals. Many older interfaces were upgraded with Medallion transformers, so older interfaces (like the 2A version) may incorporate newer transformers. Are Medallion transformers compatible with non-Medallion transformers? Absolutely. Will they sound exactly the same...probably not. So if you must mix vintages, I suggest replacing LF transformers in both speakers if you can.
 
6HB5 on sale through 2021-11-29

If anyone needs tubes for your Acoustat direct-drive amps, Antique Audio Supply is having a sale on NOS tubes including 6HB5. No affiliation, just wishing I had a pair of these and some panels to drive with it. No room for such a system, and no time to build one, so passing along to someone more fortunate.
[wiki=
https://www.tubesandmore.com/products/vacuum-tube-6hb5-beam-power-amplifier ]%[/wiki]