Ampeg SVT Classic Clean Power Output?

[timv]

Hi,
New member and first post here.
I used to be on ampage a lot and keep an eye on talkbass etc...
Just looking for some other techs input.
I have a Crate BV300H guitar head that I am currently doing some power tube testing in to see which type of tube puts out the most clean power.
I'm doing the testing with some GE 6550's, JJ KT88's, Winged C 6550's, Sovtek 6550we's, some peavey chinese labeled 6550's, Valve Art kt88's, and some Sovtek KT88's.
This guitar amp has the same power amp section as the SLM SVT's and I believe the same transformers, etc.
I can only get at most 220 watts clean.
I'm using a 4 ohm load box that I made out of precision 50 watt resistors and a 100HZ test signal, this is my standard setup for testing of all amps.
All older SVT's I've tested do about 270 clean, right before the wave on my oscilloscope starts to clip.
Do the newer SVT's put out less than this?
Thanks for any input.
Tim

[Enzo]

First, verify if the transformers are the same. What numbers are on them? Next are the operating voltages the same?

Next find out WHERE the amp is clipping. If the phase inverter or drive tube is clipping, there is nothing the power tubes can do to stop it.

Next look at the circuitry, including the support stuff for anything that affects frequency response. Your guitar amp may be designed to roll off at the bottom more than the SVT. I use 100Hz myself for most testing, since I can listen to 100Hz loud a lot longer than I can listen to 1000Hz. But that is a low freq. Try your tests at 1000Hz and see if the comparison stays the same.


A quick look at the schematics for BV300H and SVT CL show differences. B ith have +660 on the plates, but 300H shows 380v screens and -50 on grid while SVT shows 345 and -45. They both have split load PIs, but those run on different B+ voltages, and the load resistors on the SVT are 68k while the BV300H has 47k.

Have you verified the driver tubes are 12AU7 like they should be, and not 12AX7?

A look at the SVT shows the power amp in jack feeding the first buffer tube directly. The 300H goes through a couple op amp stages first. SIgnal levels on the two models differ through the power amp stages. Were you driving the amps via the power amp jacks rather than through the preamp?


Everyone has his own idea of what bias ought to be, but did you set up these amps to the bias the factory recommended for these tests?


Just some thoughts there.

[timv]

Hey Enzo,
Thanks for the reply.
I did a bit more testing today and figured out that the entire driver section is capable of more clean power so it is the power tubes are @ the max in this circuit.
Looking @ the schematics further I found that they actually only rate power output @ 290 watts @ 5% distortion so my values seem to make sense.
IT's funny, comparing the power supply of the crate 300 to an SVT classic, the crate has more capacitance than the SVT which is kind of strange.
I did try 1Khz and got the same output.
I am putting my signal into the return jack so bypassing the preamp.
I tried a few different 12AU7's for the heck of it but on the scope the drivers are clean all the way up to about 35 vrms on the output.

Running the GE's I tried biasing like an SVT, about 24ma per tube, and like the schem says, 115ma per side and got the same output just no crossover distortion.
This amp had blown screen resistors when I got it and I replaced them with 1K 5 watts so I may try bumping those back to the 100 ohm like the schematic says and see if the output changes.

Thanks for the input. I was mainly looking to see if anyone else had an idea what the new Ampegs put out clean.
One thread I have read said only about 250 watts clean.
My 70 with 6146's does about 360 watts clean, all other old SVT's I have had and tested do about 270 with 6550's.
Thanks again.

First, verify if the transformers are the same. What numbers are on them? Next are the operating voltages the same?

Next find out WHERE the amp is clipping. If the phase inverter or drive tube is clipping, there is nothing the power tubes can do to stop it.

Next look at the circuitry, including the support stuff for anything that affects frequency response. Your guitar amp may be designed to roll off at the bottom more than the SVT. I use 100Hz myself for most testing, since I can listen to 100Hz loud a lot longer than I can listen to 1000Hz. But that is a low freq. Try your tests at 1000Hz and see if the comparison stays the same.


A quick look at the schematics for BV300H and SVT CL show differences. B ith have +660 on the plates, but 300H shows 380v screens and -50 on grid while SVT shows 345 and -45. They both have split load PIs, but those run on different B+ voltages, and the load resistors on the SVT are 68k while the BV300H has 47k.

Have you verified the driver tubes are 12AU7 like they should be, and not 12AX7?

A look at the SVT shows the power amp in jack feeding the first buffer tube directly. The 300H goes through a couple op amp stages first. SIgnal levels on the two models differ through the power amp stages. Were you driving the amps via the power amp jacks rather than through the preamp?


Everyone has his own idea of what bias ought to be, but did you set up these amps to the bias the factory recommended for these tests?


Just some thoughts there.