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Old 15th September 2012, 03:57 AM   #1
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Default New Project - EL84 PP and preamp - suggestions welcome

Hello folks,

If anyone is interested in providing useful input on my project, I'd appreciate it.

A while back I purchased a Motorola console amp - the HS-768, which has 2 SE channels (left and right) and a PP channel to drive a bass speaker.
It looked good with the copper chassis and it tested ok, however there wasn't much volume to be had. I'm assuming the preamp that it was originally matched with was designed with enough gain to drive it properly.

Anyway I decided I was going to rewire the 2 SE channels into a PP one (adding 2 new PP transformers). The layout lent itself very well to this, so I decided to clean up the copper chassis, as it would look great all shiny. Long story short, I had no luck making it shine. It was a steel chassis with a thick copper coating and was never meant to be on show, so the plating had been corroding through to the steel in places.

So now I have a nice set of 4 x EL84 tubes and 2 x 12AX7s and I plan to rebuild the amp on a new chassis (copper plate), as well as building a separate preamp to drive it.

Amp stage:
For the power amp, I have a few options as to which topology/schematic to go with, and some of them are similar.
My output transformers have no extra taps for UL connection, so I'm happy enough to go with pentode or triode operation.

Motorola HS-768 (removing the C3,R5,C6 pi filter and C8, and tweaking the NFB by adding a cap)
SE and PP stages:
Click the image to open in full size.

MOTOROLA PSU:

Click the image to open in full size.

MOTOROLA PRE TONE:

Click the image to open in full size.

I've read good things about the Pilot amps:
PILOT SA-232
Click the image to open in full size.

Or perhaps running the Hashimoto in tri/pentode?
HASHIMOTO EL84 UL
Click the image to open in full size.

I'm planning to go with tried and trusted designs in the beginning, and will probably make some changes, try out CCS or LED bias, and upgrade my transformers/PSU as budget allows.

Preamp:
The preamp is mostly decided for me, although I'm open to changing it a little.
The original Motorola preamp was powered from the amp PSU, but I would prefer to keep the components independent, so I'll be building a power supply into the preamp.
I have already purchased a Chinese-made active Baxandall tone stack which uses 2 x 6N2 tubes. It is surprisingly well-made (i.e. somebody did their soldering job properly!) and it appears to have good quality components, apart from the electrolytics, which I will be replacing. This tone section is based almost exactly on the circuit given by P.J.Baxandall in the NegativeFeedbackTone.pdf (easily found via google).

Click the image to open in full size.

One thing I want to do is incorporate a Tone-defeat switch, so I'm thinking one of the following methods:
TONE-CONTROL_F.NACHBAUR-DEFEAT
Click the image to open in full size.

TONE-CONTROL-DEFEAT
Click the image to open in full size.

Obviously, I don't want to get a big change in gain when the switch is flipped, so I'm unsure exactly how to do that (i.e. how much feedback to switch in via resistor/divider). The switching in the op-amp circuit looks simple to me (the simple addition of a dpst switch would be nice and easy).
(I'll leave the arguments for and against tone controls for another time thanks)

Oh, and I've added some DC heater regulation and remote-control boards with motorised volume pot, so it'll be a bunch of PCBs underneath, as opposed to the power amp, which I am going to wire point-to-point.

I've done plenty of time as an electronics engineer but I'd appreciate anyone's opinions, as this is my first tube amp build.



Last edited by Seth1334; 15th September 2012 at 09:47 PM. Reason: Fixing image links
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Old 15th September 2012, 04:13 AM   #2
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For PP EL84 the goto amps are, IMO, El Cheapo, Baby Huey, & Red Light District. 2 of those use 12AT7 front ends.

I'd forget about the Pilot with its paraphase splitter, i'm partial to a CCSed LTP (1st 2 above), we have a class A triode EL84 version of El Cheapo sitting on a Scott donor that is a fantasic amplifier.

dave

PS: your 1st 3 pictures yield a page that says access denied
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Old 15th September 2012, 04:30 AM   #3
rmyauck is offline rmyauck  Canada
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The Paraphase splitter is really liked by many and Scott used it in their first integrated. I've read nothing but great reports on that Pilot. Floating Paraphase I believe actually?

Today Leben uses that circuit in their expensive PP EL84 amp.

One improvement I read for the Pilot is to replace the 12AU7 with a 6CG7/6FQ7 and the 12AX7 with a 12AT7.

Another mod to try for improved sonics and output tube life is fixed bias. For a low cost way you could use Dave Gillespie's EFB version, or the traditional with a small transformer.

I can't get any links to open.

Randy
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Old 15th September 2012, 04:39 AM   #4
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Quote:
Originally Posted by rmyauck View Post
The Paraphase splitter is really liked by many
Not by me. Everytime we have deparaphased an amp it has gotten better. A lot better in the last instance, a Grant Fidelity Rita.

dave
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Old 15th September 2012, 04:49 AM   #5
rmyauck is offline rmyauck  Canada
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Maybe the Floating was better as it came later and was used on the more expensive amps. The Magnavox used the non floating. I'll have to try the floating someday.

Dave which Paraphase did you convert? What was the improvement?

Thanks!

Regards!

Randy
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Old 15th September 2012, 05:13 AM   #6
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Quote:
Originally Posted by rmyauck View Post

...which Paraphase did you convert? What was the improvement?
Less colouration, alot more DDR.

dave
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Old 15th September 2012, 09:28 AM   #7
jane is offline jane  Norway
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Quote:
Originally Posted by planet10 View Post
Less colouration, alot more DDR.
The 510k and 470k+470k resistors divide the anode voltage of the lower 6SL7 and feed this voltage into the grids of 6SN7 and 6SN7. The DC grid voltage of the lower 6SL7 will be ~(Va x 2/3) and the grid voltage of the upper 6SN7 will be ~(Va x 1/3).
Drawing error?
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Old 15th September 2012, 09:44 AM   #8
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Drawing error?
Possibly. Drawn from a hand sketch by the boy genius next door. It's not like that anymore.

dave
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Old 15th September 2012, 09:50 PM   #9
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Default Fixed image links

Looks like twitpic doesn't like hotlinking.
The schematics should show up now!
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Old 2nd October 2012, 02:33 AM   #10
heyraz is offline heyraz  United States
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Seth,
I recently converted a Fisher 481-A 6bq5 console amp to a Stand Alone Amp with some very generous help from the guys on this and other forums. I love it! Best cheap amp ($125 w/out tubes) I've ever heard. The conversion consisted of adding a couple of resistors and jumpers to make up for the missing tuner and mpx components. The only thing I hadn't foreseen was how much heat those resistors would be emitting and the cumulative damage to nearby components that would eventually cause. My next step is to relocate those resistors to an area of the chassis with better ventilation/radiation properties away from sensitive components.
In the meantime I picked up a Pilot 232 amp which should arrive any day now. I've read nothing but rave reviews about that amp, some even consider it to be one of the top three 6bq5 vintage amps ever made. If comparing the size of its' iron to that of the Fisher is any gauge, I anticipate a great amp. My plans for the Pilot are the same as the Fisher, get it to run stable, then think about modifying for improvement.
Regarding the Paraphase splitter, the only thing I can say about that is I don't understand it yet. I want to keep the Pilot "sound" so I don't plan on changing it. The only alteration I foresee is replacing the 12au7 with 6fq7 based on the seemingly unanimous preference for the later. Otherwise, I will address any individual issues as they arise, much as I did with the Fisher.
If you pick up a Pilot 232, I would be very interested in sharing ideas to our mutual benefit. I've done that before with other members on other components and had great success.
Good luck on your search.
Rich
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