Power too low

[Leevi]

I'm wondering why I get so low power rate from a AB763 Bandmaster clone.
The preamp consists only of the normal channel. Power amp is according to the schematic.


http://www.thevintagesound.com/ffg/schem/bandmaster_ab763_schem.gif


Another exception is the plate voltage which is much lower than in the original one but
the bias is set "hotter".


The power before clipping is 11W.


Anyway according the data sheet below the power should be about 26W.
The sound is OK.


https://frank.pocnet.net/sheets/049/6/6L6GC.pdf


Some values:


Plate voltage 341V
Screen voltae 341V
Cathode current/tube 50mA


Power measurement setup:


RMS digital volt meter
Oscilloscope
Signal generator 1 kHz sine wave used
8 Ohm power resistor


Measured AC voltage before clipping 9.4V => 9.4^2/8 = 11W


Leevi

[kagliostro]

Plate voltage 341V
Screen voltae 341V
Cathode current/tube 50mA

With which load ?

Franco

[Leevi]

8 Ohm load
Leevi

[Leevi]

I think it's a load resistance issue.
I'm using hammond 1750P as an OT
http://www.hammondmfg.com/pdf/EDB1750P.pdf


I have set the load resistance to 6800 Ohm i.e. by connecting 8 Ohm load to 4 Ohm output
which gives the 11W. If I connect the 8 Ohm load to 8 Ohm output (i.e. 3400 Ohm load resistance)
I get about 21W power out.


/Leevi

[kagliostro]

There is something odd on the data Hammond share for that OT

on the datasheet they write 3400ohm primary

http://www.hammondmfg.com/pdf/EDB1750P.pdf

on the Catalog Page for the 1750p is indicated  126, 500, 2,000

http://www.hammondmfg.com/pdf/5C_GuitarLine.pdf

 


---


Here the Application Data for the 6L6-GC





with your B+ (340v) seems that the correct load will be 6.600ohm and give ~22-24W power with that load...........

Franco

[Leevi]

Yes indeed but I think the data sheet has a correct value since the OT is for JCM 900 i.e. 2xEL34.


Another more tricky thing is how to interpret the data sheets.


In the data sheet below there is an option of p/p AB where the load resistance is 3800 Ohm which is
is close to my case (3400 Ohm). At least the amp sounds best with this setup and the power measurement gives 26W.
The plate voltage is a bit higher now, 355V.


http://www.radiostation.ru/tubes/6L6GC-GE.pdf


/Leevi

[Leevi]

There is something wrong in the 1750P documentation.


I put 10VAC to primary (plate-to-plate) and got the following values


2   Ohm pin #9     0,45V => ratio 22.2
4   Ohm pin #21   0,19V => ratio 52.6
8   Ohm pin #16   0,32V => ratio 31,3
16 Ohm pin #14   0,68V => ratio 14,7


It seems that only the 16 Ohm output is correctly marked.


So
pin #9 is 8 Ohm
pin #16 is 4 Ohm
pin # 21 is 2 Ohm


This clarifies the mismatch with the power and load resistance.


/Leevi

[kagliostro]

I would like to read an answer from Hammond about that

may be they have a defective batch of transformer where pin #16 and  #21 are swapped ?

Franco

[Leevi]

The Hammond documentation is correct!
It was my fault since I had interpreted the secondary pins in wrong order.
I even removed the OT in order to see the labels.
Shit happens;)


/Leevi