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Old 10th October 2008, 09:43 PM   #1
cwujek is offline cwujek  United States
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Default Zenith stereo, EL84 SE. What to do...

This amplifier was my very first tube amp, and it's the mother amp that got me interested in audio when I was all but 17. It used to be in a large console w/record player, now all that is left is the chassis. I remember hearing it for the first time, it was quite magical. Sounded stunning. A few months later one channel stopped working, and one EL84 started red plating. Thought a OPT blew, but switching the OPTs between channels still left me with one tube red plating. Tried switching tube and also a few components, but never really spent a good amount of time troubleshooting this amp. I realized today that this amp is better than that, and I now intend to rebuild it completely in a new chassis. Tubes used: 2xEL84s, and one each: 5Y3, 12ax7, 12au7.

I know there are a lot of circuits out that come highly recommended, but I want to preserve some aspects of this amplifier. I will use the power transformer and 5Y3, and would really like to also use the OPTs. Anyone know where I can find out more about the transformers? I searched online and also in the forum with no luck. They have the following codes:

PT: 95-1837 117 V.A.C 50-60CY. 1386208

OPTs: 95-1650 1386152

Nice beefy PT, the outputs look somewhat smaller than what I'm used to... I guess it's only one EL84, how big do they really need to be....

Any other comments welcome, here are some pictures:

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Old 10th October 2008, 11:08 PM   #2
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Nice little amp. It's probably 12A?7 -> tone controls -> 12A?7 -> EL84 output. Schematic might be something like this:

Click the image to open in full size.

Test the power supply can cap for DC leakage. If it's still good, you can save a couple bucks by neglecting to replace it. Maybe consider stripping out all the rest of the parts, and rebuild an RH84 in the chassis.
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Old 10th October 2008, 11:53 PM   #3
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Another option is to follow the DECWARE SE84 model and triode wire the "finals". Eliminate the nasty tone control circuitry. The tube complement is reduced to a 5965, 2X EL84s, and a 5Y3.

Triode wired EL84s yield only 2+ WPC, but the sound is GOOD.
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Old 12th October 2008, 12:09 AM   #4
cwujek is offline cwujek  United States
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Quote:
Originally posted by Ty_Bower
Nice little amp. It's probably 12A?7 -> tone controls -> 12A?7 -> EL84 output. Schematic might be something like this:

Click the image to open in full size.

Test the power supply can cap for DC leakage. If it's still good, you can save a couple bucks by neglecting to replace it. Maybe consider stripping out all the rest of the parts, and rebuild an RH84 in the chassis.
I've decided I'm not going to keep this in its original chassis. I just finished a copper/walnut chassis that I will be using instead. Power supply will naturally be completely rebuilt.

Quote:
Originally posted by Eli Duttman
Another option is to follow the DECWARE SE84 model and triode wire the "finals". Eliminate the nasty tone control circuitry. The tube complement is reduced to a 5965, 2X EL84s, and a 5Y3.

Triode wired EL84s yield only 2+ WPC, but the sound is GOOD.
Yes, the Decware SE84 is a possibility, as is the RH84. I still am not sure what design I will choose, although it seems like the Decware is a better choice. Did they use Edcor OPTs in the Decware SE84? I'm still interested to know what people think of using the original iron from this amp. I know the PT is fine, but are the OPTs decent? Anyone use old Zenith OPTs?
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Old 12th October 2008, 01:54 AM   #5
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Yes, the Decware SE84 is a possibility, as is the RH84. I still am not sure what design I will choose, although it seems like the Decware is a better choice. Did they use Edcor OPTs in the Decware SE84? I'm still interested to know what people think of using the original iron from this amp. I know the PT is fine, but are the OPTs decent?
The SE84's O/P trafos are of good quality and they are custom. There is a single 6 Ohm speaker tap and 10 KOhms are presented to the "final", when an 8 Ohm load is used.

Edcor offers some high value stuff, but (AFAIK) none of their stock models meet the requirements for this job. At $95 each, 50 mA. DC rated "iron" from ElectraPrint wound to the spec's given above is suitable.

If you want to bring the B+ rail voltage up, switch the rectifier type. The filament draw of the 5R4, 5V4, and 5AR4 is 2 A., like that of the 5Y3. So, overstressing the power trafo is not a concern. The types are listed in declining forward drop order.

Given their lack of heft, I suspect Zenith's OEM O/P trafos are no better than so-so.
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Old 12th October 2008, 06:37 AM   #6
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Try the RH84 circuit with your stock xfrmrs. You will be surprised. Yes the size limits LF responce, but the RH84 has the best LF responce of all the common SE EL84 circuits.
My magnavox SE which has been converted to an RH84 is stunning, and the little transformers are almost half the size of yours and they do a full 4 watts and dont saturate untill you hit like 15hz. I have not verified this with a scope, but I know what saturation sounds like and its not nice

Anyway, I would definatly try the RH84. It is simple,and due to the unique local feedback lowering internal resistance it offers triode like performance AND sound at pentode output levels, as well as very good damping for a pentode. It also only has one capacitor in the signal path- the coupling cap.
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Old 12th October 2008, 10:21 AM   #7
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I've got a small PP 6V6 amp that came from a Sears Silvertone console,it's got surprisingly good bass,given the small size of the OPT's.

Give the stock iron a shot,you can always upgrade later.
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Old 13th October 2008, 12:18 AM   #8
cwujek is offline cwujek  United States
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Good advise. I'll hide the OPTs under the chassis, so if I do go with anything larger, no ugly previous mounting holes will be visible. I'll give the RH84 a shot, but it looks like I will be needing a choke. Shouldn't the choke be able to handle more than 50ma? Recommendations on a choke, or anyone have a used one for sale? Thanks for the words of wisdom thus far... I'll post pictures of the bare chassis soon.

Chris
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Old 13th October 2008, 03:32 AM   #9
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Shouldn't the choke be able to handle more than 50ma?
Yes,

Figure ~ 38mA for each EL84 plus ~ another 2 - 3 Ma for each driver half (going from memory for my original). I used a Hammond 158Q in mine which is 5H 150Ma.

I am now rebuilding mine using the same choke, I only started the rebuild about 3 years ago
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Old 13th October 2008, 04:26 AM   #10
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A very similar amp -- called the dumpster diver -- and a pair of 40-1197 BD-Pipes got me on the road i'm currently on.

OPTs very similar to these reside in one of out RH84 buillds.

We typically use a hammond 10H, 100mA choke for every pair of EL84s.

dave
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