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Old 6th January 2011, 12:40 AM   #1
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Default Help me make use of these transformers

So I acquired a Chinese PP el34 amp a while back on the cheap and have been really pleased with the sound, unfortunately some reliability issues have taken it out of commission a time or two. So, I'm tossing around the idea of trying to repair it appropriately (as opposed to just putting band-aids over issues) or in true DIY fashion taking the thing apart and using it for parts for something else.

I figure the transformers are the most expensive part of any build so I'd like to get some ideas of what might be possible with what I've got. Both the power and output transformers are big and beefy but should I decide to pursue this path I'll probably build the amp to allow the option to change them out in the future (should the need arise).

Anyway, specs are as follows:

PT is a toroid
Dual 115 v primaries
165V @ 1 AMP (two green wires)
6.3V @ 8 AMPs (two blue wires)
35V @ .1 AMP (two brown wires)

OPT
6K P-P UL @40%
55 Watts
4,8 Ohm taps
10Hz-40KHz +- 1dB (advertised specs, seems pretty wishful thinking to me)

The goal of this project is to learn about the build process and how things really work. I'm still a super newb so this is going to be a challenge for me but we all start somewhere, right? I would like to land somewhere in the 30+ wpc range and would love to be able to use the Russian 6s3p-e tubes I've got on hand but could also go with something different (the stock Chinese el34's would be possible as well).

So I've read this (super long) thread and while I don't understand most of it, it seems very interesting:

6L6GC AB2 Amp

I'm also curious about the El Cheapo Grande:

El Cheapo Grande

Last but not least, the el34 based Musical Machine (which I've heard lots about and I know boywonder has built one but I don't personally know much more about this one).

I'm open to any other ideas but keep in mind I'm new and don't have mountains of test equipment to build something totally unproven; actually I've only got my trusty Harbor Freight multimeter, but I do have 4 of them if that counts for anything!

Thanks in advance for any help/assistance.
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Old 6th January 2011, 11:42 AM   #2
Ian444 is offline Ian444  Australia
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I would start by measuring the OPT's, to determine the reflected primary impedance. In my experience a Chinese OPT is a completety unknown factor until measured, the results may or may not please you, but at least you will know what you have to work with. For example, an XD500 EL34 PP music angel OPT's, very big in size, measure 12K primary for 8 ohm on the secondary. Mingda MC34B designed for 6L6, 5K2 for a 4 ohm load and 3K9 for 8 ohms. Makes you wonder. Hence my first suggestion, measure your OPT's to determine what they are and go from there.

The PT will need a voltage doubler, that's OK, nothing wrong with that, and the B+ will end up well over 400V so 6P3S may or may not work, probably need a regulated screen supply which is a good thing anyway, but the original Chinese EL34's will probably handle the voltage more easily/safely. I have never had a problem with the sound of Chinese output tubes.
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Old 6th January 2011, 01:10 PM   #3
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Try this one out for size. DCPP, AKA Engineer's Amp & Red Board Amp.

It is very flexible and while you may need to add a second low voltage transformer, the 35V used as a doubler will get you a bias/-CCS supply; the 165V will translate into about 445 volts, enough for some serious output power. The whole of the amp is on one circuit board. The only additions you'll need to make are the two voltage doublers. Read the thread thoroughly, this basic board has been pushed from it's original 18WPC to over 240 WPC!

Stuart
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Old 6th January 2011, 02:25 PM   #4
jrenkin is offline jrenkin  United States
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If you like the sound, why not learn by tracing and drawing out the original amp's schematic. Then you can post it here for review, mod recs and component upgrades. After that you could even rebuild it P-P with any appropriate mods and have the amp as it it wants to be!
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Old 6th January 2011, 08:22 PM   #5
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Thanks for the thoughts so far. First up, sounds like a good idea to check the actual impedance on the OPT. Is there any easy way to do that? I did some reading online and the simplest method I found required a variac and a multimeter; I don't happen to have a variac. Any other options?

The red board from Mr. Millet has got my attention, been following that thread for a while, but I really want to avoid a PCB. Not that I don't like PCB's, I just hate PCB mounted tube sockets.

As for the third rec, I do have the schematic (or at least what is reported to be the schematic):

http://www.diyaudio.com/forums/attac...-el34_6dj8.jpg

There was quite a bit of discussion/confusion/argument as to what tubes actually belonged in which slots. Lots of back and worth over what could be subbed for what. I did like the sound of the amp when it was working so maybe I should just try and rebuild what I've got P2P and see if I can't sort out some of the confusion. Any thoughts on the schematic as shown?

Anyway, like I said, I really appreciate the thoughts so far and please keep them coming, particularly on how to measure my OPT transformers and opinions on the schematic.
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Old 6th January 2011, 09:02 PM   #6
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Easy way to test transformer turns ratio. Connect the transformer secondary 8 ohm and 0 volt taps to a low impedance transistor amplifier's output. Connect an FM tuner (if you don't have a signal generator) and tune between stations. Use the interstation noise as a signal. Turn the transistor amp up enough to get an even integer voltage (to make the calculation easy), say 1 or 2 volts. Once that is set and stable, measure the voltage from plate to plate leads on the transformer primary. Divide the plate-plate by the applied voltage at the secondary, and there is your turns ratio.

To get the primary impedance, multiply the speaker impedance by the square of the turns ratio. For instance, if you determine the turns ratio to be 20 to 1, and you are connecting an 8 ohm speaker to the 8 ohm tap, then 20 squared time 8 is 3.2k.

Be very careful when doing this. Observe proper live voltage working requirements, as you could generate serious and life threatening voltages on the plate-to-plate leads if you turn up that transistor amp a little too much.

Stuart
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Old 15th January 2011, 06:08 PM   #7
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Bigjppop,
I don't mean to thread jack and wasn't sure to post here or in the Suppo thread of this amp but I recently acquired the same amp and was wondering if you could elaborate on the specific reliability issues you were having with the amp?
Quote:
Originally Posted by bigjppop View Post
So I acquired a Chinese PP el34 amp a while back on the cheap and have been really pleased with the sound, unfortunately some reliability issues have taken it out of commission a time or two.
I am very new to the world of tub amps and mod electronics but I have some difficulties with the schematic for one it appears the schematic show two 12AX7 tubes but mine only has one (and actually I am using a 12AT7). It also appears to be missing the 110 230 switch.
Quote:
Originally Posted by bigjppop View Post

http://www.diyaudio.com/forums/attac...-el34_6dj8.jpg

Any thoughts on the schematic as shown?
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Old 18th January 2011, 02:38 AM   #8
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Quote:
Originally Posted by TriSel View Post
Bigjppop,
I am very new to the world of tub amps and mod electronics but I have some difficulties with the schematic for one it appears the schematic show two 12AX7 tubes but mine only has one (and actually I am using a 12AT7). It also appears to be missing the 110 230 switch.
Spent some time this weekend looking over the schematic and reading up on electronic symbols. Realize now that the Scheme does show just one 12AX7 tube (same tube different pins). Also I opened up your link to the schemeatic and the 110 230 switch was clearly shown. I had opened a link to the other schematic from that thread which does not have the switch on it. Sorry for messing up your thread.
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