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In Reply to: RE: BEZ T3A-3 amp 2A3 posted by BillWojo on July 16, 2017 at 14:24:33
Now that I have had some time to listen to this I find that the upper end gets congested on anything short of simple music. Could this possibly be worn out 2A3 tubes?
On that note, how are the vintage RCA and other US tubes compared to the newer tubes.
So far, this has nothing on my MC40 monoblocks. Looks like there is one cap in the audio circuit, between the EF37A and the 2A3. Pretty simple build.
BillWojo
Follow Ups:
Most folks find that Chinese output transformers severely limit the performance of a single ended amplifier.
Hopefully this works. BEZ, from what I gather builds transformers. Here is a picture with the top cover off. Decent looking outputs in there. Doesn't mean that they are great, I know.
I just don't know if worn out tubes will make the upper frequency's all congested. Sort of smeared together.
BillWojo
Bill,Go to T3B-7 and T3B-8 models. Photos and schematics(electrograms) are available. Check against your amp. Amend as needed.
Edits: 07/27/17
I don't think the anemic pentode will help the high frequency response of that amp.
An anaemic pentode shouldn't smear the high frequencies together. I'd expect it to be more like a gradual roll-off of the high frequencies. No smearing in my system - doesn't sound overtly rolled off either, but neither does it sound artificial, etched or pushy like a high GM, high mu IDHT. Go figure.
Is the EF37A interchangeable with the 6K7?
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I'm also thinking of one of those older Hammond gapped single ended output transformers that had some resonant behavior that was very audible at somewhere around 8K IIRC. The same type of problem could be happening with this amp, and it'd be pretty easy to measure with a scope or sound cart and FFT software.
Thanks mah, I believe those schematics are pretty much a match. Even the PT has a 2.5V tap for the 2A3 tubes.
RCA inputs in the same place, the power supply caps are beefed up somewhat but the internal pics are very close.
I'll probably put the MC40's back in my system, they are hard to beat. In the mean time I'll be on the look out for a set of 2A3 tubes to try out. I would find it hard to believe that BEZ would ship an amp out sounding this way. It's either the tubes or something else is wrong. I need to open it back up and take a closer look.
Take care.
BillWojo
The TB3-8.
It seems it takes an EE to find the hard stuff. Only joking fellas.
Edits: 07/27/17 07/27/17
When I look up info on the EF37A it comes up as a high gain tube with a nice straight curve. Sounds like it should be a good tube for it's current usage. Have you had experience other wise?
BillWojo
The 6K7 is supposed to be directly interchangeable with the EF37A tube. I believe that when new this amp shipped with 6K7 tubes and someone upgraded the tubes.
If the high's were rolled off than I might suspect transformers but it's not "roll off" I'm hearing. It's the inability to play anything complex at the mid high range. It just smears together.
Since 6K7 tubes are dirt cheap, I might get a pair to test with. I'll also look into testing the 2A3 tubes.
Thanks
BillWojo
When you describe it like that, it sounds a bit to me like what I hear when a tube is worn out. This is odd, however, as you say you have it on both channels.
The other possibility is that the amp is just running out of gas and distorting.
One of the things I don't like about this amp is the lack of a bottom plate to easily access the wiring. Once the top cover is off the tube sockets and wiring are mounted to another plate that lifts up but not much due to the short wires going to the RCA jacks on the side of the chassis near the front. I need to get back into it and see how this thing comes apart.
In the meantime, I can easily measure any voltage going to the tubes. I have a complete set of tube socket test adapters that fit between the tube and the socket with a ring of test points.
Looking at the schematics above, it looks like filament is simply to both filament pins and the other voltages are referenced to ground? Let me know if this would be of help.
Thank you.
BillWojo
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