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

[Kagliostro]

Many Thanks for the Layout

The PI isn't the standard Paraphase, it is the Floating Paraphase

you can read about this PI here

http://valvewizard.co.uk/paraphase.html

K

[brewdude]

I hope the sketch helps.
I suspect that between the schematic (which I believe to be accurate) and the photos you will figure it out. If you have question, I'll be happy to help.

Honestly, I will be totally thrilled and genuinely honored if someone else were to build anyone of my peculiar and odd designs.

[didit]

Nicely done. '50s era Gibson amps are fun inspiration for departure. Curious on the selection process of signal pentode, subbing 5725 for 6SJ7. Any hard reasoning, or just a conveniently available 7 pin?

Best .. Ian

[brewdude]

Thanks didit,
The reasoning behind the 5725...

Actually it was the 5725 that started me down the rabbit hole that led to this amp. I followed a link someone posted about a sale on these cute little tubes and bought 25 of them for like a buck each. I am a big fan of the 6sj7 and thought at this price it was worth checking out.

Anyway, I had been mulling over ideas about how to use them. I wanted to try an all seven pin build. I wanted to try using two pentodes in the preamp. And some members of the wattkins amp forums had been using the 5725 in a GA-40 style amp based on a later schematic. As I searched the web for info on the GA-40,I ran across the earlier schematic and noticed the use of pentodes in the preamp. This schematic also incorporated a big ass choke as a CLC filter before the OT center tap (I have a fascination with choke input PS's). With these discoveries I was compelled to pursue this design--with much help from the members of ppwatt.com.

So far I am very pleased with the results.

I have heard rumor of an amp design that utilizes all 5725 tubes for pre and PA. With so many acquired so inexpensively I've thought about trying a low wattage build of this nature, but lack the expertise to design it on my own.

[martin manning]

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.

Typical Tweed amps had two back panels, upper and lower. I was thinking that the upper back panel would just cover the chassis' open side, so you could easily reach the power switch.

[brewdude]

I'm confused by your tweed panel description.
If mount it with the volume and tone facing up and the open part of the chassis facing rear, the power switch would be on the underneath side--as it is wired right now.

[martin manning]

Yes, that's it. Should be easy to get to the power switch. You'd need a vent in the top, so maybe do two handles, Vox style.

[Colossal]

Beautiful build!

[brewdude]

Thanks Colossal,
I appreciate the kind words.

You all make such wonderful, beautifully executed builds with very well thought out details. I am honored to be acknowledged.

I hope to get some sound clips to share at some point.

[brewdude]

Martin,

Nice and quick work on the tweed style diagram--thanks. Perhaps, your are right and it would not be as terribly difficult to reach as I imagined.

[brewdude]

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 messing around a little with this amp today. I found that the pair 6005's I have were pretty far off in current draw from each other. So, I put the 6AQ5's back in. I also rolled a couple different 6C4's through the PI and settled on a Phillips JAN.

I was playing with the PA bias trying to warm it up some and recalled that I had installed a resistor to limit the maximum voltage of the VVR. Bypassing this resistor put the plate to cathode voltage at about 254v--just above the rated maximum--but, it sounded a bit better to me. When I also paralleled a 330R with the 150R resistor the sound was probably the most lively an dynamic with both of these mods, but it put the plate dissipation just above the maximum 12W. I think the higher voltage was slightly better than the lower cathode resistor, so I will likely replace that VVR max voltage resistor with a jumper and leave the 150R for now. Also, with the high heat (and therefore high electrical demand) I have found that the AC voltage supplied through my wall outlet varied from 117v during the day to 120v in the evening. In other times of the year it is often 122 or 123v. So, it's probably not wise to set the bias at near 100% with lower AC supply.

On a side note, I also tried the EL95's, but found that I was uncomfortably over their plate dissipation limit of 6W with the 150R... they probably need the 180R.

[tubeswell]

Way cool. I want to hear it.

[brewdude]

Harvest has kicked into high gear in Napa. I'll likely be working overtime everyday for a while. I promise to get some sort of sound sample up when I can. Even if it's a simple iPhone recording.

[tubeswell]

Harvest has kicked into high gear in Napa. I'll likely be working overtime everyday for a while...

Cool. Vineyard work? (One of my brothers is a winemaker here in NZ. I get all the stories)

[brewdude]

I'm work in production--cellar lead. I usually figure I can't make any plans from middle of August to middle of November.