Tube DIY Asylum

It is possible.

128.125.179.129

sser2

In Reply to: RE: Heeey... could we adapt this to a 45? posted by Vinnie on October 04, 2007 at 14:23:48

71A and 45 have same amplification factor and Miller capacitance, so 45 should work in this circuit too. However, if full benefit of increased power is to be obtained, B+ and idle current should be increased, and there will be much more heat generated by the cathode resistor. Will also take 1.5 times of driving signal swing.

Instead of parallel 6SL7 I would use a tube with more driving power, something like one section of 12AT7 or 12AZ7, choke load it, and feed it from a separate supply. Voltage gain will be about the same.

The most vulnerable spot of this circuit is cathode bypass cap. There is nothing remotely good in this value, so provided all parts are as good as it gets, this cap will dominate the sonic signature.

Follow Ups:

RE: It is possible. - drlowmu 23:40:24 10/04/07 (0)
Hi sser2,

Correct, a 12AT7, etc can ONLY be used with a choke load because it's mu is not ideal for a two stage DC amp otherwise. A mu of 70 is the minimum you want, with a set of film caps, highest quality, across it's cathode. A mu of 100 is even better. Seventy or higher gives you the good old "jump" factor.

Contrary to our shared experience, high driving power is NOT needed or wanted in this type of circuit, basically a Loftin White. Anywheres from 1/2 to 1 mA will do fine, with GOOD wire in the whole amp, and particularly between the direct couple is a must.

See the Ansano Loftin White amp on the web, somewhere I saw the schematic. It uses 1/2 mA and half a 12Ax7 as I recall. Yes, it is surprising, but that type of mU and low current tube is ideal in a L-W sser2. ( ez jim ) See the Loftin White write up in Sound Practices, from the two Italians, an EXCELLENT article to have.

Jeff Medwin
RE: There is nothing remotely good in this value - Caucasian Blackplate 22:11:28 10/04/07 (1)
50uf/200VDC auricaps should work just fine in that app...
For less money, some of the offerrings from SCR, Jantzen, and Parts Express should also do the job nicely.
-Paul
- Jantzen Caps @ $14.00 ea (Open in New Window)
- RE: There is nothing remotely good in this value - drlowmu 23:21:43 10/04/07 (0)
  You will want to bypass any large films, with a .01uF Dynami Cap and maybe something between. Also, you need to compute the uF value, for 4 to 8 HZ being -3 dB.
  
  Jeff Medwin

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