From rfrie–(at)–etcom.com Thu Feb 13 21:16:05 CST 1997
Article: 38537 of alt.guitar.amps
Newsgroups: alt.guitar.amps
Path: geraldo.cc.utexas.edu!cs.utexas.edu!howland.erols.net!ix.netcom.com!rfries
From: rfrie–(at)–etcom.com (Robert Fries)
Subject: Re: 68 Princeton Reverb
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Date: Fri, 14 Feb 1997 02:09:31 GMT
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Joe Rella wrote:

>Robert Fries wrote:
>>
>> rfrie–(at)–etcom.com (Robert Fries) wrote:
>>
>> >I just got one of these, tube chart says AA764.
>>
>> >The earliest schematic in the Weber and Pittman books is
>> >AA1164.
>>
>> >Anybody got a schematic for AA764, or know the difference, if any?
>>
>> >RF
>>
>> In a breech of net etiquette, I’ll follow up my own post…
>>
>> My AA764 tube chart calls for a GZ34 rectifier, the AA1164 schematic shows
>> a 5U4; I suppose this might be the main (only?) difference.
>>
>> RF
>> *****************
>> Robert Fries
>> 415-988-9475
>> rfrie–(at)–etcom.com
>> *****************

>Robert, go with the 5U4GB. The main reason is the GZ34 will drive your
>plate voltages up past 460V! Absolutely kills the 6V6GT’s in no time.
>I recently picked up a fairly clean ’66 Princeton Reverb with the same
>AA764 chart, and there was a GZ34 in there. I tested all the tubes and
>discovered the Sylvania 6V6′s were weak, but not quite dead yet. They
>looked fairly new (nude base variety) Popped in a matched set of similar
>Sylvania 6V6GT’s and the vibrato turned very weak. Since that works off
>the bias, I decided to start taking a few measurements, and discovered
>the plate voltages were around 463V. Popped in a new 5U4GB, measured
>again, and was down to 418V, with that nice strong vibrato. I surmise
>that the reason for the quick 4 month run from AA764 to AA1164 was the
>choice of rectifier tube for the above reason.

Joe,

You got me curious, so I experimented with various rectifiers:
(this table reads best with a fixed-pitch font.)

Rectifier Plate V Bias V Idle ma Power Line v
Mesa 5AR4/GZ34 401 -29.4 23/25.5 17w 120.7
RCA 5U4GB (old) 389 -29.5 20.3/22.3 13.1 121
Nat’l Elec. 5AR4 (old) 378 -29.6 17.3/19.0 10.2w 121.5
Svetlana 5U4G(near new) 388 -29.4 19/21.7 12.25 121.3
GT SS 411 -29.5 25.3/28.3 18.6 121

The plate current measurements are in left tube/right tube form. The power
measurements were taken using a 1khz sine wave, measure the P-P voltage with
a ‘scope, and use the formula P = (Vp-p^2)/8*R, into an 8 ohm 50 watt
resistive load, which yields RMS power in watts.

It seems my ’68 puts out a lot less voltage than your ’66. Worst case
idle plate dissipation is with the SS rectifier, and at 11 watts, even that
should be OK with the old RCA 6V6GTA’s that are in there. I think I’ll
stick with the Mesa 5AR4/GZ34.

All the above measurements were made with all tubes installed; when I checked
the voltage with the SS rectifier and NO power tubes, I measured about 460V at
the plate pins on the sockets. Did you have tubes installed when you did your
measurements?

Comments?

RF

*****************
Robert Fries
415-988-9475
rfrie–(at)–etcom.com
*****************