Few watt RMS

[grtamp]

I finished my amp and I seems it's not much powerful for a 100W ( twin PT and OT w/4x6L6).
I checked with oscilloscope and I take about 70Wrms.
66Vpp with 8Ohm load. Plate voltage on 6L6Gc= 443V Bias about 37.5/37.8
I try different power tube like svetlana winged C , but it's the same!
any suggestion!
Thanks

[dobbhill]

What stage is distorting @ 70watts? Pre-amp or PI or output tubes??
Please give us more info about your testing setup.
D

[ToneMerc]

The Twin OT is only around 85W RMS. With your voltages it looks like you are biased at 67%.

.67*25W= 16.75W, 16.75/443 = 37.8mA


TM

[grtamp]

the probe is connected to the load ( resistive load ) therefore I measure the Vpp on secondary of the OT.

[butwhatif]

is 443v at idle, or full power?

[grtamp]

443V at idle , no signal input.

[pamaz67]

unfortunately there are lot of Power transformers that have a less than desirable regulation, therefore the voltage they deliver on full power request is sometimes 10 - 15% less than the idle one.
Myself, have been facing similar problems with some Twin replacement transformers coming from one of the "well reputed" supplier.
As a result, I had to replace all of these irons with some other ( better ) stuff.
The only thing I can imagine, is to use them on low powered amps. Or use them to lock the door open....
Lets' say that I have four power transformers rated for fender twin that are going to be used on 50 watters. The amp was not able to do more than 65 w at 5% distortion with that stuff.

Paolo

[llemtt]

8-12% regulation is what's expected from a 100W guitar amp PT, Paolo is right lots of replacements trafos have terrible regulation figures (i.e. they are overrated, probably a 70W PT inside).

Idle voltage shouldalso be in the 450-460v range to be ok, 85Wrms at the onset of clipping it's the true power of a twin output stage and it's damned loud!!

Marshall output stages are totally different beasts, no clean output available, compared to fenders, but 135rms are usually delivered at 5-8% thd (distortion becomes clearly distinguishable as such).

cheers
teo

[jelle]

The great Marshall plexi amps, the really good ones to me, drop about 100V under full load.

[grtamp]

Hi guys ,I solved the problem , 4-8 Ohm tap inverted ,wiring error.
Now I can take 91 Wrms before clipping, more responsive amp and punchy!!
Thanks all

[FUCHSAUDIO]

Good call. Also, if you are using a variac or isolation transformer, make sure the AC line is not sagging under load. I had that problem which caused a 150 W amp to put out 80 watts, as the AC line loaded from 120 to 95 through the variac....whoops. 91 Watts is fairly decent for most Twin Iron.

[grtamp]

Hi , yes , I use always a variac on my workbench.

[FUCHSAUDIO]

Then you want to make measurements at 120 V under load. Make sure you lower variac when you plan to kill the signal, so the line won't jump up. Other choice is to perform your RMS testing direct into the wall, once the amp is done.

[M Fowler]

I use a variac on my bench but I am thinking I should also have an isolation transformer and some kind of AC filtering unit. What do you guys use or prefer?

Mark

[hitchcaster]

unfortunately there are lot of Power transformers that have a less than desirable regulation, therefore the voltage they deliver on full power request is sometimes 10 - 15% less than the idle one.
Myself, have been facing similar problems with some Twin replacement transformers coming from one of the "well reputed" supplier.
As a result, I had to replace all of these irons with some other ( better ) stuff.
The only thing I can imagine, is to use them on low powered amps. Or use them to lock the door open....
Lets' say that I have four power transformers rated for fender twin that are going to be used on 50 watters. The amp was not able to do more than 65 w at 5% distortion with that stuff.

Paolo

paul at mercury magnetics told me that the heyboer power transformers voltages brown out under load... and tried to talk me into his PT... is this what were talking about?