how to tell if an amp is operating in class b

[black_labb]

hi guys
ive picked up another holden/wasp amp to use for bass. it runs on 4 6550's like the one i already have, but its power section is a bit different (havent mapped it out or anything, just noticed that there are more diodes in it). when i picked up the amp i spoke to the previous owner for a while, and he said that it could put out 307W of clean power (im assuming that he had a tech test it). when i pulled out the chasis there was a sticker with a date (january '78) and it also said 270W as well as 3 letters (vtv which i assume is a model designation or similar).

im pretty sure that you couldnt get 270w or 307w of clean power from 4x6550's in class AB. how will i know wether it is class b? will there be any signs of it when i draw out the schematic? i am only really doing this out of curiousity, as i like to understand the things i am working with.

im not planning on putting it onto an scope as i dont have one (and i dont have a dummy load that will take the 200-300w). would AB2 also be a possibility for these numbers? the power transformer is quite a bit larger than the other wasp i have, which suggests that they may have needed more power, but i have heard that holden/wasp more or less just put what they had available to them together, and didnt really have steady supply lines for some parts.

thanks for any help

here are some pics of the 2 holden/wasps
http://smg.photobucket.com/albums/v6...holden%20wasp/

[jrfrond]

The most I can see pulling from a quad of 6550's in ANY mode is 200W, and that would be in ultralinear mode with about 600V on the plate. Now, 6550's are real good at this, but NOT for 270W-307W RMS.

Because of their origin, are you SURE these weren't designed with KT88's?

[black_labb]

im quite sure that they are designed for 6550's, as the company was making the amps with 6550's for a while. they used kt88's for a bit before they went to el34's and then 6550's.\


not sure of the voltage on the new one, but the old one is running dead on 700v (thats with 30 yr old filter caps).
the fender 300ps bass amp runs 4 6550's at 300w (and the 400ps runs 6 at 400w), so its possible.

[Steve Conner]

Hi guys

The Fender 300PS and 400PS made that extra power using Class AB2, with a special driver circuit to supply current to the power tube grids. They used a 6V6 (a 6L6 in the 400PS) feeding through an interstage transformer. There are other ways to run AB2, like driving the grids with extra tubes (12AU7s or even EL84s) wired as cathode followers, which is what Ken Gilbert's BAGA does.

I don't see any sign of this circuitry in the pics of your Holden Wasp guts, so I can only assume they are just ordinary Class AB1. I hardly think it matters whether they give 200W or 300, either way they'll be loud as all hell.

I'm not sure whether it's a wise idea to use Class AB2 with new production tubes, anyway. The 300PS and 400PS had a reputation for being pretty hard on their tubes. That Rich Koerner guy said that new production 6550s won't cut it, and only NOS GEs could take the punishment, which makes a 400PS very expensive to run.

I heard rumours that the new Russian tubes all have gold-plated grids that are very easily damaged by grid current. I actually have the iron from a 300PS, but I can't decide whether to use the interstage tranny or not. I don't want to buy a sextet of Tung-Sol reissue 6550s and wreck them in 5 minutes.

[black_labb]

so it was ab2, makes sense. im pretty sure that the 4th 12ax7 is a post pi gain stage, but it could be wired as a cathode follower, but i doubt it. i'll do some research then see how it turns out.

[jrfrond]

Hi guys

The Fender 300PS and 400PS made that extra power using Class AB2, with a special driver circuit to supply current to the power tube grids. They used a 6V6 (a 6L6 in the 400PS) feeding through an interstage transformer. There are other ways to run AB2, like driving the grids with extra tubes (12AU7s or even EL84s) wired as cathode followers, which is what Ken Gilbert's BAGA does.

I don't see any sign of this circuitry in the pics of your Holden Wasp guts, so I can only assume they are just ordinary Class AB1. I hardly think it matters whether they give 200W or 300, either way they'll be loud as all hell.

I'm not sure whether it's a wise idea to use Class AB2 with new production tubes, anyway. The 300PS and 400PS had a reputation for being pretty hard on their tubes. That Rich Koerner guy said that new production 6550s won't cut it, and only NOS GEs could take the punishment, which makes a 400PS very expensive to run.

I heard rumours that the new Russian tubes all have gold-plated grids that are very easily damaged by grid current. I actually have the iron from a 300PS, but I can't decide whether to use the interstage tranny or not. I don't want to buy a sextet of Tung-Sol reissue 6550s and wreck them in 5 minutes.

I agree. There is no way I would put anything but GE-NOS 6550's in a 300PS or 400PS. I've worked on a number of these over the years, and I can tell you that they run the tubes balls-to-the-wall. New Russian tubes might just put on a light show in one of these babies. And yes, Rich Koerner is THE dude for these amps.

[black_labb]

just out of curiousity, do kt88's work alright in the 300/400ps? wouldnt that solve the issue, or are they not suitable for another reason.


edit: i doubt that the amp would be putting out the 270/307w that i was told. i was very sceptical when he said it put out 307w clean power, and thought he meant 207w due to getting mixed up. when i saw the sticker inside saying 270w i had second thoughts, as thats 2 sources saying it puts out close to 300w.

im not dissapointed that it doesnt put out 300w, as i know that it wouldnt make a big difference in sound. im not opposed to a bit of break up either, if i get there.