View Full Version : Help needed on BIG tube amp

05-08-2010, 08:46 AM
An E bay thriller.....

So i have a pair of 4333's which I just LOVE... and I thought it would be cool to build a tube amp..

So I won the following.. from an estate sale back east, from the head of the machinist union boss..
A circa 1960's ish kit, which is partially complete... Alot of time was spent on the chassis & wiring, but the wiring was not completed and I do not believe this ever made audio... but here are the parts I have to work with.

Peerless R800A power transformer ( working ) with 860 VAC output ( No load ) 6 volt, and
5 volt outputs and that works!
and the tubes all light up, none of the DC filtering is wired so no problem getting B+ to the tubes.. it's not there, YET!

The outputs are a bit of a mystery, they are 5 wire primaries and 4 wire secondaries ( 4,8 16 ohm outs ) the ONLY marking I can find 6W24-VG and 1005421 so I'll run some DCR testing to get a ROUGH idea where I'm at.

There are 4) 5881 ( Tungsol ) 2) 6AN8's 2) GZ-34 / 5AR4's and 2 OA2's!!

The filter section are 3 dual 80MFD @ 350 and 1 4 section 20 @450 ( Which all measure within 5% of spec and 2 were never soldered onto ).

There are also 2 fuse style diodes 1N1084's that come off the rectifier stages of the 5AR4's and go NO WHERE!

So I start testing the parts today, and try to find a base schematic ( Like a Dyna Mk III ) to start as a base wiring point.

The internal point to point wiring is 70% complete, and of course there is no schematic.

So any input would be greatly appreciated while I formulate a plan for world domination...

Thanks in advance


05-08-2010, 08:58 AM
This one may have been modded.

However, it might be in your best interest to rebuild it with new circuit design and parts. You already have the Peerless output trannys (If they are good, you got your moneys worth!) Peerless tranys have a great reputation for great sound.

I think with PP 5881's you are looking at anywhere from 25 wpc to 40 wpc. Anything more than that is possible with those tubes and trannys, but the sound would not be optimum for Hi-Fi so to speak. Commercial duty amps use 6L6 PP which are similar to the 4881s for up to 60 watts.

I'm not sure this amp would handle the bottom end if you play at louder than normal levels, you might need a larger SS amp for the bottom and use this amp for the upper frequencies if bi-amping.

Nice output trannys!!!

Regards, Ron

05-08-2010, 09:10 AM
I'm not sure I'll use the 5881's I also have a stash of 4 KT88's... BUT I'm just in the what works and what doesn't stage... I think this was a home brew so skys the limit for a redesign...

right now I'm flipping the A/C switch ( upside down ) and getting the pilot light in circut...

I would like to get 60+ watts out of it... I did a fisher 400 a while back and it doesn't have enough poop for me...

I know the OA2's were a cold cathode regulator and act like zeners so I'm reading the spec's on that next.. the rectifier tubes didn't even have the DC out ( pin 8 wired ) to anything... so I'll see what gives there... and theres something on pin 3 ( which should be a N/C ) which would have fed the 2nd pair of Soild state rectifiers ( but only one of those was wired ).. it's quite the mystery, but I love puzzles )


05-08-2010, 09:29 AM
I'm not sure I'll use the 5881's I also have a stash of 4 KT88's...

If you can't use the 5881s, I might be interested in buying them!

I agree with the posters above. There is a lot of nice stuff there and for the price you did very well. I'd start building it from the ground up with the circuit of my choice.

Good score!

05-08-2010, 11:41 AM
5 wire primary on output transformer is likely an "Ultra-Linear" mode where the screens get about 70% of the plate voltage.

Just google Ultralinear tube output

05-08-2010, 11:43 AM
I believe the next 3 items
1) What the HECK are these 2 transformers for output ( make model serial # ).

2) What base schematic? Dynaco Mark 3 ( 2X )? Or where is there a good collection brand X,Y,Z amps to build from?

3) OA2 regulation do i need it, or will it just be a pretty coke bottle?

As far as caps & resistors.. C/D green drops & there all 5% carbon I 'll look into the different " audio purist " variety.. but I think on the initial build the Cornell Dubliers will be fine...

2 of the big cans are 30% out of tolerance....
all the green drops are within 2%


05-08-2010, 12:03 PM

it looks like page 8 is a good reference for me.....


05-08-2010, 03:56 PM
...with those 6an8s in there I'd keep looking at Dynaco Stereo 70 schematics. The amp is also fixed bias and has AC and DC balance pots which all sort of point in the Dynaco direction (it's most likely not a Williamson amp).

Also have a look here for something similar:

Those output transformers have part numbers an awful lot like Heathkit, Scott, and Fisher amps. Perhaps made by Chicago or Stancor? With the extra leads on the primary UL is a possibility. Before gutting it I'd try to sort out the circuit that's in there first. I'd say you got a good start at an amp for the price! :)

05-08-2010, 07:23 PM
None of the DC bias stuff is wired.. and the more I look at it... the guy changed gears 1 or 2 times on the design. The left doesn't match the right for parts & layout... and there are no choke coils...

It seems I could go with either design Dynaco, Williamson.. I'm not LOCKED into anything..

It would be nice to implement all the existing test points etc.. but not that big of a deal to me.
( But I defer to the wisdom of this forum )

I had a dryer to replace today & sent the car to have some car audio installed...

So tonight...
A) test the tubes
B) DCR for outputs transformers
C) Sniff out various schematics

and off to an early ZZZZZ........

05-09-2010, 08:09 AM
I have an example of a tube amp design which shows the use of the 0A2 and uses 5881/6L6/6CA7, etc. It can be adapted to other types as well 6550, KT88, KT90, 6B4G. The power supply is tube regulated and if you are interested I have a PDF of my article on the tube amp as well as building a tube regulated power supply...this gets you very good bass response for one thing. The files are large, so PM me with your email if you would like copies.

Steve Schell
05-09-2010, 09:53 AM
Vigman you have a real learning opportunity here, at least if you would like to learn more about tube circuits. It looks like a fairly ambitious homebrew effort to me, circa early 1960s or so. If this was one person's dream amp, there may not be a schematic in existence. Or maybe it was built from a magazine article... hard to say. I would post on several other audio boards such as Audio Asylum to see if anyone can identify the circuit.

You might want to try hooking up with an advanced solder sniffin' tube maniac. I would recommend first trying to draw out the intended circuit from all the available evidence, then decide whether to use that or mod it into something else. From the careful chassis layout and neat look underneath it appears that the builder knew what he was doing.

If you do go to the effort to complete this project amp I would recommend replacing all the can electrolytics. They are old, probably dried out, and would be the weak link in reliability even if they seemed to work acceptably at first.

05-09-2010, 01:25 PM
Beowulf57... sent you a PM yes PLEASE send as much as you dare!

Steve... I have made the decision to pull all the existing resistors & caps out... I attemped to map the existing out... but there are just to many differences between left & right so we will start at bare chassis underneath.. the other thing I'm concerned about is the lack of an additional tap on the peerless power transformer for IF I need a bias circut... I assume I can add a smaller transformer underneath to do that... so any input there will be helpful.

The output transformers do work ( jamming an osc into the primary and driving a small car speaker on the secondary ) the DCR's 240 ohms across the secondary Blue to Brown ( Red is CT, to brown 113 ohms. CT/BLU and 125 ohms . blue to blue /yellow, blue yellow to red 48 ohms, red to brown / yellow 53ohms, brown/ yellow to brown 60 ohms )...

The other side is within 5% and no stinky smells from crispy parts....

So today I'm just going to clean it up...... the tubes are all " good " according to the Heathkit...
Not what I would consider a matched set.. but good enough for fire up!

Thanks for all the support!


05-09-2010, 01:55 PM
Not Oa2 but OA3's



05-09-2010, 03:04 PM
Not Oa2 but OA3's



Yes...I noticed that when I had a look at the amp. The OA3 operates at 75V with a range of 5-40mA, while the OA2 operates at 150V with a range of 5-30mA. All depends on what front end circuit you decide to use.

05-09-2010, 04:41 PM
Now a heap O reading : )


05-11-2010, 08:17 PM
But none the less.... I have new TS strips & fiberglass hook up plates, similar to what was in there.

I think I will go with the Dyna MK 3 to start ... and use a separate transformer for the bias circut...
I will also have to go the MAGIC STASH at work to see if we have some old school chokes...

I have more than enough real estate to mount many many parts in this chassis...
Maybe this weekend I'll start the build of the PS sections.

I'll also look about for some different tube regulators I'm not sure why there is only 2....
Isn't the idea to have 1 per rail to act as a zener after the 5AR4's and filters, caps etc..


09-16-2010, 03:17 PM

Lava lamp optional


09-16-2010, 03:26 PM
What circuit did you end up going with?

Regards, Ron

09-20-2010, 10:37 AM
beautiful !