Twin PT for 50 watt build?

[crbowman]

Ok, got a real newbster question for you guys.
I’m building a 50 watt 70’s style amp using Natchanis’s 60’s style chassis.
The chassis appears to be cut to accommodate a Twin PT. So the magic question is, aside from price and weight considerations, is there any downside to using a Twin PT for a 50 watt amp? Upside??
As always, you guys input is greatly appreciated.

[David Root]

It won't sag at all when cranked, will be pretty stiff. If that's what you want, fine, but it's overkill.

Why not make it a 100 Watter? But then you'll need a 100W OT.

[crbowman]

Thanks David.
I had thought of that, but the chassis I have is pre-cut as a two-holer so I really want to stick with 50 watts. I guess I could use a resistor instead of a choke if I wanted to loosen it up a bit.

[Funkalicousgroove]

it'll be fine, I do it all the time- ALL of the fender iron is underrated for it's application, Leo was cheap!! You may increase the resistance after your choke to get your preamp voltages down a bit-

ALSO, A JTM45 PT works great for this as it is the same cutout as a Twin but a 50W trannie, and you can use the 5v taps to run your relays-

[crbowman]

Once again I benefit from Leo's frugality!
I'll look into that JTM45 PT as well.
Thanks Funk!

[UltraHookedOnPhonix]

I put a Twin PT in my 50W Overtone. Works great!

Here are my Voltages:

B+1 (Plates): 457V
B+2 (Screens): 455V
B+3 (PI): 438V
B+4 (OD): 307V
B+5 (CL): 299V
V1 (1st Stage): 201V
V2 (2nd Stage): 196V
V3 (3rd Stage): 198V
V4 (4th Stage): 206V
PI Plates: 299V (non-FB side), 292V (FB side)
Heaters: 6.74V (3.37V per side)

Note: when using a Twin PT, you may see an elevated heater voltage since they’re rated at 6.5V. Then you can do something like this (read under, “Voltage considerations”):
www.freewebs.co.uk/valvewizard/heater.html

Here’s a pic of my amp:
[IMG:743:1023]http://i205.photobucket.com/albums/bb17 ... TwinPT.jpg[/img]

[Structo]

So did you use diodes in your heater supply?

[crbowman]

Thanks U-HOP!
That's a pretty interesting link.
Heater voltage was one of the main things I was concerned about.
Nice work by the way....

I put a Twin PT in my 50W Overtone. Works great!

Here are my Voltages:

B+1 (Plates): 457V
B+2 (Screens): 455V
B+3 (PI): 438V
B+4 (OD): 307V
B+5 (CL): 299V
V1 (1st Stage): 201V
V2 (2nd Stage): 196V
V3 (3rd Stage): 198V
V4 (4th Stage): 206V
PI Plates: 299V (non-FB side), 292V (FB side)
Heaters: 6.74V (3.37V per side)

Note: when using a Twin PT, you may see an elevated heater voltage since they’re rated at 6.5V. Then you can do something like this (read under, “Voltage considerations”):
www.freewebs.co.uk/valvewizard/heater.html

[UltraHookedOnPhonix]

So did you use diodes in your heater supply?

No, I'm thinking of getting one of those voltage regulator racks, so I'll get consistent performance. Sure, I'll consistently get a heater voltage of 6.74V but I don't think .4V too much is considered pushing it, YMMV.

Thanks U-HOP!
That's a pretty interesting link.
Heater voltage was one of the main things I was concerned about.
Nice work by the way....

Thank you crbowman!
Yeah, there's some cool things on that site. He's also got a new book out on designing tube preamps that looks neat.

[jaysg]

Note: when using a Twin PT, you may see an elevated heater voltage since they’re rated at 6.5V. Then you can do something like this (read under, “Voltage considerations”):
www.freewebs.co.uk/valvewizard/heater.html

I'd go with Schottky's in your case.

[David Root]

Well, I stand corrected. For these circuits I guess that also explains the sometimes high uF plate caps eg #124 that HAD used? Stiffer is better with PT and plate caps?

[Structo]

When you say "plate caps" are you referring to coupling caps?

[David Root]

No, the plate power supply caps.

[crbowman]

Just noticed the JTM45 PT has no bias tap. I'm assuming I can just tap off of one leg of the HT prior to the rectifier and sub in a higher value resistor for the 1.2K resistor that feeds the bias supply..? I think the JTM45 uses either a 150K or a 220K. Is this correct? Any other changes need to be made?

[David Root]

Given that a lot of 100W Dumble ODSs were made with standard Twin iron, what would folks recommend in overrated iron, both for 6L6GC and for 7581A/7027A, ie higher Vp??

How about a Music Man 4-130 PT with a 25W Zener on the CT? Somehow I think the best answer is custom iron.