Clone Sorta Fender Pro Reverb advise?

Riverboat

Member
Messages
28
Hi all, I am back again needing some info. I had to do some redesigning since my last post and the amp is working better with far less clipping of the signal but I am not getting the amount of volume that I think this design should be putting out. For those reading about the amp that I have built, I will try to describe what I have. I have built an amplifier using the Fender Pro Reverb AA165 schematic. It has only one channel with reverb and no vibrato. The preamp channel is coupled to the reverb circuit using the standard .02 capacitor. With this design, the volume was not near what I thought it should have been with P-P 6L6GCs as finals. I have read that the Vibrato channel usually didn't have the volume level the Normal channel had in the original amplifiers, so I took it a little further and also coupled the preamp channel into the PI circuit via a .047 cap and a 220k resistor connected to the 220k reverb mixer resistor like the original Normal channel. In essence, I’ve created a “vibrato” channel with only reverb and the addition of a Normal channel coupling capacitor to the PI also. I reversed the output leads of the reverb transformer so the signals at the two 220k resistors right before the .001 PI coupling capacitor would be in phase. The maximum volume level before clipping came up but still does not seem to be as it should. I am including measurements that I have made with and without a 1K sinewave at 200mv. applied, in hopes that someone with technical expertise can give me some advice as to what I am doing either right or wrong.

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INFO:
Power Xfmr – P-TF22798
Output Xfmr – P-TF 22848 – 4 ohms
Choke – 22699

Oscilloscope probe X1

No Sinewave input:
Power input – (119.5 Vac)
PT xfmr output – (350 Vac)
Standby on B+ – (+490 Vdc)
Standby off B+ - (+435 Vdc)

2- 6L6GC Outputs
Output xfmr CT – (+435 Vdc)
Output tube Plates – (+433 Vdc)
Output tube Screens – (+434 Vdc)
Output tube Cathodes I – (48.4 ma)
Output tube Grids – (-40 Vdc)

Phase Inverter 12AT7
PI-2 – (+63 Vdc)
PI-3 – (+98 Vdc)
PI-7 – (+65.2 Vdc)
PI-1 – (+226 Vdc)
PI-6 – (+226.7 Vdc)
V at junction of 82k/100k – (+423 Vdc)

Signal Applied to preamp input: 200 mv.
PI-2 – (+61.6 Vdc)
PI-2 – (6 Vpk-pk)
PI-7 – (+63.8Vdc)
PI-7 – (5 Vpk-pk)
PI-1 – (+215 Vdc)
PI-1 – (45 Vpk-pk)
PI-6 – (+215 Vdc)
PI-6 – (45 Vpk-pk)

1-6L6G –5 – (-40.3 Vdc)
1-6L6G –5 – (45 Vpk-pk)
1-6L6G –3 – (+410 Vdc)
1-6L6G –3 – ( HUGE Vpk-pk)
2-6L6G –5 – (-39.8 Vdc)
2-6L6G –5 – (45 Vpk-pk)
2-6L6G –3 – (+407 Vdc)
2-6L6G –3 – ( HUGE Vpk-pk)

With only a 1X probe I could not measure the Vpk-pk on pin 3 of the output tubes but it was way over 45 Vpk-pk. I think I might be able to use a 10meg resistor in series with the probe and measure it as a 10X. I have a 1/10/20/100X probe on the way.

OUTPUT SIGNAL at Speaker Jack: ( 45 Vpk-pk)

I was using a 8 ohm passive load on the 4 ohm output so I would expect the output to be lower. So, is 45 Vpk-pk correct with this load? I have two output jacks wired in parallel so I can install two 8 ohm speakers. I went back and installed another 8 ohm passive load which gave me 4 ohms. Here are my calculations as to what the Power Output should be with both 8 ohm and 4 ohm:

Vrms = .3535 x Vpk-pk
Vrms = .3535 x 45
Vrms = 15.90

8 ohm load calculation:
P = Vrms2/R
P = 15.90 x 15.90 = 252.81/ 8
P = 31.60 W

I was thinking that I could get at least 40Ws out of this amp but the calculations don't give me that amount.

Outputs biased at 70%
Plate voltage 433 Vdc
Tubes 30W

.70 X 30 = 21
21 / 433000 = 48.4 ma

4 ohm load calculation:
P = Vrms2/R
P = 15.90 x 15.90 = 252.81/ 4
P = 62.20 W

The 4 ohm calculation just don’t seem right to me and I think I’m missing something in my way of thinking especially since the tubes should not be able to produce more than the 30W each since they are biased at 70%. When I went from 8 ohms to 4 ohms, the output sinewave did not increase so I used the same Vpk-pk to recalculate the Power Output with a 4 ohm load. Help… straighten me out!!
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TweeDLX

Member
Messages
3,756
Can you post a schematic of what you actually have? If I'm understanding it right, I'm thinking the cap/resistor combo you added did nothing. In the original, that 220K resistor was a summing resistor. Each channel had one. The Vibrato channel has an extra gain stage after the reverb that boosts the signal. It doesn't look like you have that. Might explain why you don't have the volume you're expecting. Can tell anything without a schematic though, so this is all conjecture.
 
Messages
525
In your past thread about this amp, other members asked you to post photos of the amp. That would be a great idea if possible.
 

Riverboat

Member
Messages
28
Took me a little while to create the schematic in AutoCad but finally got it done. I had to scan the plotted printout as AutoCad does not save in .jpg, .bmp, etc. I am including a link to the schematic for the amp I built and a link to the Fender AA165 schematic that I used.

http://s1139.photobucket.com/albums...on=view&current=SortaCloneFenderProReverb.jpg

http://s1139.photobucket.com/albums...view&current=FenderPro_Reverb_aa165_schem.jpg

I redesigned the circuit from my early post to include the .047 & 220K around the final reverb gain tube in order to mix in some lower frequency signal to the PI. I may be having a problem with this since the reverb circuit would normally be out of phase with this addition. I did reverse the output leads from the reverb transformer so as to get the reverb signal in phase with the signal through the .047 & 220K that I added but did not take into account the dry signal through the 10pf/3.3m filter to the gain tube which would still be out of phase. DUH!!

Couldn't I just remove the 10pf/3.3m circuit so the .02 signal will go to the reverb tank which is now back in phase with the original signal from the preamp and have the .047 & 220K dry signal to the PI. I'll have to take a look at that to see what the Vp-p out of the reverb gain tube is and compare it to the dry signal.

Anyway, back to my original question. The voltages and the Vp-p output calculations. I hope these schematic help explain the text more clearly.

I'm probably just confusing you guys with my thoughts....

Whatcha think?
 

TweeDLX

Member
Messages
3,756
I emailed you some info. In your schematic, you've included the summing resistors for a two channel amp. Yours is a single channel amp. You don't need those. Remove them. If you're confused (sounds like you are) look at a schematic for a Princeton reverb. Basically, you're building a PR with 6L6GC's and a Long-tailed pair PI. Get the wiring right first, and then worry about calculating voltages.
 

Riverboat

Member
Messages
28
A huge thanks to TweedLX (Mike) for some very useful information. I really appreciate it. What a great place to come to when we have a problem.
I will be implementing some changes to my circuit and will be back with further info as to what I find.

For those interested, here are my calculations as of today before changes.

Input: 200mv @ 1 kHz
Output: 42Vp-p

Vrms = 0.3535 X 42
Vrms = 14.847

Load: 8 ohms
P=(14.847)2/8
P=220.43341/8
P=27.55 Watts

Load: 4 ohms
P=(14.847)2/4
P=220.43341/4
P=55.10 Watts

Sam
 




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