paraglow rebuild

[deepgroovebiz]

Greetings,
Many years ago I was in Philly visiting Mike at his old address. He was thinning out his gear and I went down to buy some Allen Organ Amps. He kept offering more and more gear, some I grabbed and other gear I REFUSED . We were about go back up when he spied a pair of Bottle Head amps on the shelf.  I vaguely remember seeing him building them years ago in his living room, I thought to myself. How much? “$50” he said, they were covered in all sorts of debris on them the tranny covers were rusted, the brass frames needed refinishing etc. All easy work in my mind.
When I started sorting them out they had all Magnaquest iron, pgp8.1 power transformers and even a Magnaquest filter choke and nickel outputs. I recapped them refinished them and added them to my collection. I figured them to be Para glows. For $50 he should have at least included some cobalt iron.

So time marches on and the itch to reconfigure them kicks in. But this time I have some new 1/8 thick top plates for the amps. They are cut out for a pair of TL 404xl 5k (yes 5k is a little high for 2a3) and no I do not want to make them into 45.  I was going to build up some 5 inch high walnut frames, so I can add some Rod Coleman Regulated filaments and the required trans formers. I also want to use some poly ammo caps for the power supply

So the big question? Leave them dc coupled with c4s? I do like the variety of 12at7s. Cap couple them and use a choke input for the power supply. Then I could use a mu follower not a srpp or just plain old plate resistor.
So what is a gurl to do? I do appreciate a more aggressive sound bass slam etc.
Thanks
Chas

[Paul Birkeland]

The first thing to do is to cut a physically larger top plate.  At least a 10"x10", if not even larger.

The TL404 isn't the best choice for that particular circuit, but it's a common misconception that there's some magical output transformer impedance for each tube, and that you can't use any other impedances.  For instance, the Paramour/Stereomour/Stereomour II run 4K output iron. Having said that, you likely have a perfectly good set of Magnequest plate chokes and output transformers, so why not use those?

300V plate to cathode and 45mA of plate current will get you close enough to be able to use the TL-404.   

For the driver/power supply, a choke input power supply (not quite critical inductance) would help to drop the available voltage down so that you can cap couple, and half of a 12AT7 C4S loaded will be a good choice.

[deepgroovebiz]

Thanks

the plates are cut already, so with a 5 inch tall chassis there is plenty of room.

So the question that remains is  dc coupled vs cap coupled?
I am more concerned with sonics rather then specs for the input tube topography.


THE tl 404xl is an amazing transformer that just sits on my shelf, so this is a great opportunity to put it in service.
Mike was insightful enough to band it to a base that has the same foot print as the TFA2004/TFA204

May I ad that the amp has magnaquest EXO 03 plate choke and I also have a pair of HY nickel grid chokes floating around

[Paul Birkeland]

I am more concerned with sonics rather then specs for the input tube topography.

Are you saying that you believe that these are mutually exclusive aspects of amplifier design? 

[deepgroovebiz]

I am honestly waiting for Paul Joppa to offer his opinion on the front end topography!
Chas

[Doc B.]

PJ may be out of the country. I believe his current rate for design consultation is $125 per hour.

[deepgroovebiz]

The body of the post was some history of the amp
At the end of the post was my question

"So the big question? Leave them dc coupled with c4s? I do like the variety of 12at7s. Cap couple them and use a choke input for the power supply. Then I could use a mu follower not a srpp or just plain old plate resistor.
So what is a gurl to do? I do appreciate a more aggressive sound bass slam etc."


SINCE I have seen Paul's constructive input on the original circuit design I was looking for his opinion on the original circuit
Paul also helped Mike trouble shoot the amp

[Doc B.]

Good luck with your project.

[Paul Joppa]

Well, Dan is right; I am in Florence, Italy at the moment. Away from my notes and the lab, I can't do more than general commentary.

First, it's not clear what you have. (The only Bottlehead product I recall that used the EXO-03 was the ParaBee, and only the Paramour family used the 12AT7.)

IIRC, the TL404 is rated for 2 watts, so you won't get the bass slam from higher power using a 2A3 (or 300B). It works well with a 45 (or a 6DN7!). You could use a 2A3 at a 2-watt operating point, and probably do that as direct coupled; it would dissipate less and last longer but give the same power as a 45.

The PGP8 makes too much voltage for a cap-coupled 2A3, unless you use PBs suggestion to make a hybrid semi-choke-input power supply. Really, it's best suited to a 300B.

Hopefully that helps! I'll be back in a few weeks.

[deepgroovebiz]

Thanks for your  response
Mike always spoke so highly of you (I have been doing business with Mike since 1992)
So you got passed on with a hero status
Since there was no direct way to contact you, I started going through your posts for clues to different front ends
Then I decided to post
I wonder what the mutt is?
It definitely has a pgp8.1 with a small chassis

I have the tl 404 extra large core it dwarfs the EXO 36NI
So thanks for the info but I was hoping u would catch the xl part 
The 204 xls are reportedly good for 4 watts
With choke input the power transformer should  put me in the 300 volt range, so cap coupled is feasible

From what I read Dr. B et al spent hours voicing the dc circuit.

So stick with the dc or use something more exotic like a mu stage?

Thanks for all your help

Chas

[Paul Joppa]

With typical SET circuits, the output power is mostly determined by the available voltage and the transformer impedance. High impedances require higher voltages. Examples:

A normal 2A3 with 250v plate to cathode into 2500 ohms gives 3.5 watts. A 5000 ohm transformer will give about 2 watts.

I run the 2A3 at its maximum rated plate to cathode voltage of 300v into 4000 ohms, which gives about 4 watts. Into 5000 ohms that would be about 3.2 watts.

With a 300B, the PGP8 can run 350v plate to cathode into 3000 ohms for about 8 watts output. Into 5000 ohms that would be about 6.5 watts. (I have posted a DC heater circuit for the PGP8 with 300Bs.)

There is a design for the Robin Hood iron (5000 ohms) which I put onto Mike's forum a few years ago.

Add in the necessary bias voltage and the DC voltage drop of the plate choke to get the power supply voltage. For direct coupled, I do not like to run the driver voltage any lower than 175v. Many others do run it lower, but it produces greater distortion that way.

Sonically, direct coupled is better; if you cap couple use the best cap you can find to remain at the level of the TL404. Direct coupling requires a way to trim the driver plate voltage, which will otherwise vary with the tube and with aging - it's kind of a high-maintenance deal. The "monkey on a stick" circuit is more stable; I believe I've posted that as well.

Ping me again after mid-Ocober if you want to pursue this discussion.

[deepgroovebiz]

I found the circuit
AH Jack E use to wax poetically about the ultrapath
Thank you so much

Chas