Pultec Tube amp

[DaveP]

Hi all,

I'm in the process of building a Pultec EQP-1A, with the layout designed and built from scratch.

I made the power supply and make-up amp first and tested it before going on to the passive section (I've already breadboarded that ok).  I've used a Sowter 9330 interstage and 9530X output iron.  I used a Chinese 12AX7 from RS and a 40 year old used Brimar ECC82 for the output (it had already seen 30 years use in my HIFI !  Voltage balance of the tubes was within 2%.

I wired it old school with a heavy busbar for earth and point to point without using turret boards (don't like all those wires picking up hash.

I measured +28dB into 600 ohms load before visible clipping, that's 20V!  The amps output Z worked out at 57 Ohms.

The gain of the amp was 17.3 dB not including the interstage.

What astonished me was the frequency response -0.2dB at 10Hz then flat till 20kHz where it was -0.2dB it was -0.56 at 30KHz, -0.9 at 40KHz, -1.3dB at 50kHz, -3.5dB at 100kHz and -15dB at 200kHz.

I used a 1.5nF/9.1k zobel across the interstage which gave the best square wave performance and the above figures.  There was no hint of instability and no need for caps across the grids of the ECC82's either.

I think the famous low-end on the Pultec must owe a lot to the amp, and the top end of the Sowter iron speaks for itself.
best
DaveP

 

[riggler]

point to point without using turret boards (don't like all those wires picking up hash.

Do you have any pics of your building style? I love seeing how people do things.
No pics == didn't happen!

 

[isophase]

wow !
i would love to see some photos
also have to get started on the passive Eq

 

[DaveP]

Hi Riggler/Isophase

Do you have any pics of your building style? I love seeing how people do things.
No pics == didn't happen!


I can take pictures, but not until next wednesday cos I'm building it at work.  Basically it's symmetrical just like the schematic with wires the same length as far as possible.  This style is fiddly to build and has to be carefully thought out so that you can always solder the next component as its a 3D layout.  Obviously its crap to service but with modern components I'll be dead before they need replacing!  The amp is built over the tube sockets so it all fits in a rectangle about 4"x2" the TX is just south of this.
DaveP

 

[DaveP]

Pultec Pictures: Work in progress

Isophase and Riggler wanted to see some pictures:-

I made the metalwork myself, can't buy cheap nice cases in the UK.  I adapted a ready made ,cost about $15.
Here's the links on Photobucket.

If you guy's are interested in layout, then this is my Fairchild so far

I'll post more results when the Pultecs finished
best
DaveP

 

[Kingston]

Some really excellent tips for all builders there. Heavy gauge wire and shielded cable where appropriate, with easily readable and not constricted layout.

I especially like how you've handled the ground bar. What type of wire is it?

 

[pacemaker]

wow !!
amazing work pure DIY in vintage style,
what a beauty !
Looks like a old cadillac !

 

[DaveP]

Thanks Kingston and pacemaker,
nice to know its appreciated.

The Busbar wire is from RS part numbers 355-041 or 390-527 they call it tinned wire.

all the best
DaveP

 

[eskimo]

Yup, extremely nice indeed! My type of build right there, not that I could pull it off, but the aestethics and way of aproach is right down my lane.

 

[lassoharp]

Nice work!  Like the busbar ground and nice compact yet neat p to p wiring in the amp section.

Very good on the chassis as well.  When you say you made the metalwork yourself - shearing the raw sheets and braking?  Threading?  I can see it being good for 2 or 3 other popular projects too. Excellent DIY!

 

[DaveP]

When you say you made the metalwork yourself - shearing the raw sheets and braking?  Threading?

Hi Lassoharp,

I don't have sheet bending gear so I use ready made boxes from CPC (UK arm of Farnell).  I cut these slightly over-size with a padded electric jigsaw then I file them by hand down to the line. Finally I either pop rivet, or screw the parts together.

I use the steel cases of old computer power supplies to make magnetic shielding to stop the power supply radiating hum all over the sensitive circuitry.  If you keep the power supply up one end and the input down the other, then the layout kind of designs itself.  My Earthing arrangements are virtually Star Grounding with the heaviest current flows nearest the mains socket, all the screened signal cables and in/out XLR's are grounded to the chassis and that is grounded only at the power socket.

Finally, I always used DC supplies for the heaters and I run them in screened cable too.
best
DaveP

 

[isophase]

man, i am totally blown away. i love your design !!!!
the only point to point wiring stuff i've done so far is a 50 watt bassman (inside an old broken Musique Industrie amp head) and a little 2 watt practice amp/preamp.
and to prove you guys that it really happened, here are some proof photos





the little practice amp is from a kit sold by musicding in germany (i have to be honest on this one) even though i added a line out connector that was not in the original schematic, i still took some hints from the laying out that was proposed with the amp. so this one is not my design (or laying out i should say)
as you can see, my point to point wiring skills are far from what you just showed to us, but hay! these are not pulects !
soon i hope
i also built an "all black gssl" which you can find a couple photos of around here if you type "all black gssl" in the search fonction.
cheers

 

[DaveP]

That's good Isophase,

If you use a few cable ties it makes all the difference to the layout, they not only make the wiring look tidy but they stiffen the wires by mutual support, this stops them moving too much in transit and breaking off, important when you're gigging and moving the gear about.

Is that a 15" speaker?  For guitar, 4x10" speakers sound great (like original Bassman) or even a couple of 12" would be better.  A lot of the sonic characteristics of the Bassman circuit will be lost if the speaker doesn't have the top end ability to reproduce them.

I made my first tube amp in 1970 but I'm still learning from this forum, it's invaluable, keep up the good work.
best
DaveP

 

[abbey road d enfer]

Is that a 15" speaker?  For guitar, 4x10" speakers sound great (like original Bassman) or even a couple of 12" would be better.  A lot of the sonic characteristics of the Bassman circuit will be lost if the speaker doesn't have the top end ability to reproduce them.

Guitar amps come in all sorts of textures and flavors. As much as I love 10's in my Custom Vibrolux Reverb, I don't like them in a Super. I love the 12 in my Blues Jr, Amped Reverborocket, Vox AC30CCX but not that much in a Twin. I also have a Vibrosonic reverb, which is basically a Twin chassis with a 15". As much as I hated it with the original Cerwin Vega (the type that weighed as much as pig iron and had about the same electroacoustic performance ), I love it now with a fresh Jensen Neo. And I must say a quartet of 12's in a closed-back cab is as ballsy as can be with a cranked up Marsher.
It really depends on what you're expecting, not mentioning what guitar you match it with.

 

[isophase]

thanx DaveP,
i know the wiring could have been nicer with cable ties (this was my first point to point tube project in 2004)
yes its a 15 inch speaker (celestion G15B? i forget) i agree with you a 2x12 sounds much better but it was the only thing i had available then. i made the cab out of an old 12" guitar combo, the back is open and the whole thing is really not tuned at all. i have to make an adequate box with the right volume.
best
isophase

 

[maxwall]

Incredable attention to detail and layout strategy, when I see pictures like this it
is really inspiring to get me building. thanks for sharing the pictures of crafty workmanship.
Hope to see more from your lab.

 

[creal]

wwooowww realy nice work dave.

what is the network you used for this eq?

 

[DaveP]

Bonjour Creal,

I used various schematics to come up with this

This circuit has the correct factors of RxC, as far as it was possible make them.
I also used Toko coils from Farnell which have excellent Q and the same boost as the original Pultec.
The unlabelled resistor was 620 ohms which I understand is essential (see other posts), it certainly worked for me.

cordialment
DaveP

 

[lassoharp]

Hi Dave,

Were the Toko coils all readily available at Farnell?  We have only 3 distributors in the US and most of the values are non-stock, non-back order and several are marked obsolete. So pretty much a wash as an easy to get part.

 

[DaveP]

Hi Lassoharp,

I started collecting various coils a few years ago so maybe they have deleted some values recently, but there is also a Ham Radio component company in the UK that ships worldwide.

http://www.jabdog.com/

I found these guys after I found some coils at Farnell, and I could have got the lot from them.
good luck
DaveP