All 7 pin tube amp—Video DemoClips Added 1/27/18

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brewdude
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All 7 pin tube amp—Video DemoClips Added 1/27/18

Post by brewdude »

Edit 1/27/18: I just posted a video demo.
https://youtu.be/7lTzMwUxbwc

I've just successfully fired up my new amp.
It's a single channel, no tremolo, build inspired by the early GA-40 amp but with only 7 pin tubes.

5725 pentode input
Tilt style tone control
5725 pentode recovery
6C4 cathodyne PI
Cathode biased
6AQ5 push pull PA
Choke fed screen supply
Global NFB
Parallel 6X4 tube rectifier
CLC filter
Global VVR

Edcor XPWR013 (255-0-255v @ 125mA, 6.3v @ 4A)
Hammond 159P (10H @ 125mA, 155R)
Hammond 154E (20H @ 20mA, 1666R)
Edcor GXPP10 (10k/8R)

All squeezed into a Valve Jr. size chassis
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Last edited by brewdude on Sat Jan 27, 2018 6:49 pm, edited 2 times in total.
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

So far, I'm leaning towards calling her the "Gib-lette", but have been considering alternatives.
She is very quiet as far as noise floor.
And, plenty loud at full volume.
More of a cleanish amp until near maximum gain.
The VVR works very well, though I may raise the value of the lower limit resistor.

I have a 150R/10W cathode bias resistor at the moment which puts the 6AQ5's at around 39mA plate dissipation each, or about 80% of maximum (~244v plate to cathode). I clipped in a 330R/10W parallel to the existing 150R for an equivalent total of ~103R which brought the bias up to about 93% of max and I think it sounded better--more lively some how.

I was planning to try a pair of EL95's also, but the power went out (presumably due to the ungodly heat we are having in Nor-Cal). If they are happy with the 150R bias resistor I might keep it. If not I'm going to have to find a 100R (or possibly even 91R) replacement to get the most out of this thing tonally.

My only minor concern is that the front panel got pretty warm during my testing. I suspect the tubes mounted along the front edge are going to have that effect, especially while situated tubes down. I guess I need to devise a well ventilated tubes up cabinet to house her in.
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

I was just looking at the 5725 data sheet and realized that I may be a little over the maximum voltage.
Is the stated maximum plate voltage, the maximum B+ supply for that tube?

The Data sheet states maximum plate voltage of 180v.

My B+ supply is 210v.

V1
Plate 147v
Screen 44v
Cathode 1.5v

V2
plate 133v
Screen 38v
Cathode 1.7v

Should I be worried?
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martin manning
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Re: All 7 pin tube amp

Post by martin manning »

Not a bit. Voltages on data sheets are with respect to the cathode, so you are below the stated maximum. Nice looking amp there! I think I'd want to show off the line of 7-pin tubes by using something you could see through for the front valence in a tubes-up head, like perforated or expanded metal.
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TUBEDUDE
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Re: All 7 pin tube amp

Post by TUBEDUDE »

Nice! I like seeing an out of the box build. 7 pin tubes, what fun. A voltage starved fan could help dissipate the heat.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

Thanks Martin.
I was thinking something with an open front also.

I was originally planning a vertical 2x 8" combo cab, but I'm worried about heat now. Perhaps, I could build it as if it were a permanently attached "piggy back" cab. There are cage nuts on the sides of the chassis to bolt it to the side panels of a cabinet. Many things to consider.

I think I'll let her burn in a little and wait to finalize a proper cab until after the crazy hours of harvest are over--probably November.
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Kagliostro
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Re: All 7 pin tube amp

Post by Kagliostro »

COOL, I lke it

Image

Have you a layout that you can share ?

K

p.s.: Some time ago I was thinking to build this (with 5654 - 6AK5 preamp pentode)

Image
Last edited by Kagliostro on Sun Sep 03, 2017 4:07 pm, edited 2 times in total.
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

Thanks TUBEDUDE.

I've not built a single amp that fit well in any of the established "boxes". I've never been interested in clones. The problem is that other musicians don't really even look at them unless they are a copy of a well established iconic brand/model. Oh well, their loss--good thing I'm not trying to make living selling amps.

As for the fan, I've given it a little thought. However, it's likely going to be very tight inside the small cabinet. I'm hoping to get by with passive ventilation.
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martin manning
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Re: All 7 pin tube amp

Post by martin manning »

brewdude wrote: Sun Sep 03, 2017 3:45 pm I was originally planning a vertical 2x 8" combo cab, but I'm worried about heat now.
You might make that work if you mounted the chassis vertically, ala Tweed Fender, with ventilation on the top.
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

Thanks Kagliostro,

I would happily share a layout, if I had drawn one. I did sketch it out once at the beginning of this protect, but it was incomplete and scribbled. It has not survived the build process. Perhaps I can through something rough together...

Unfortunately, I don't expect much time over the next couple months. It appears that harvest in the Napa Valley is going to be very compressed this year and the heat wave is intensifying the urgency.
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

Martin,
I thought about mounting it Tweed style, but I put the power witch on the back panel. It would be stupidly difficult to turn it on and off.
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Kagliostro
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Re: All 7 pin tube amp

Post by Kagliostro »

If I'm not asking to much, also a pencil draw, a very spartan one, only of the Board with components values will be of great help

K

p.s.: Nice Groung Buss wire :)
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

Kagliostro,
I don't know anything about the 5654 or the 6J6, but that looks like a fun build.
Is that a James stack?
What kind of PI is that?
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Kagliostro
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Re: All 7 pin tube amp

Post by Kagliostro »

The 5654 is an HF application tube, but also some Tube Mic used this tube inside

https://www.hifiengine.com/gallery/imag ... v72a.shtml

There was a page on internet about this amp (7 Cylinder Sykora) but was deleted (now you get a 404 warning)

at the moment you can see a copy here (there are photo and samples of the amp)

https://web.archive.org/web/20161028131 ... inder.html

The PI is a Paraphase (used in some old amps and people claimed to add a special mojo to the tone of the amp)

K
brewdude
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Re: All 7 pin tube amp

Post by brewdude »

I didn't recognize the paraphase PI, it looks different then the way I've seen it drawn before.

I found the sketch I used for the board layout. Some of the values have drifted over the design process and it is rough and incomplete.

The PS filter caps for the PI and pre are not drawn in.
Definitely compare to the schematic attachment in the first post and the photos.

Regarding the ground bus, it is a length of square wire that I think purchased from AES(?)... Anyway, I put one end in a vice and the other in a drill and twisted it. This not only looks cool, but also makes it straight and stiff.
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