DR405 filter caps and repairs

[snarfusmaximus]

In cases where I can run an amp to the onset of output stage clipping...

I doubt there'll be any place where we can run it at that volume.

Damn!
Emil

[mhuss]

That's the method I use on old, stock amps. Works well, you just have to mind your fingers while measuring!!!

--mark

[synthi]

It lives!!! A very quick update (latish on Sunday evening here):

We finished off double-checking everything and then loaded the beast up with tubes today (original mullards in the pre, new JJs in the power). Fiddled around with the bias a bit, have it idling around 28-29W, with B+ around 692V.

It sounds as amazing as I remember and then some. Headroom for days. Also got a surprising amount of mojo with the guitar despite barely idling the power stage and still reaching insane volumes.

Anyway, I have been running it into the dummy loads to give it a soak and the maximum clean power output has been steadily creeping up, from about 290WRMS initially to about 345WRMS at last check - this is right before the 100Hz waveform starts to fatten up and flatten off at the top. Hoping it will creep up a little further still, but all things considered that's not half bad. I need to be careful though, as I only have 400W of dummy load and even that last check was enough to make think they were about to burst into flame. Will be sure to employ some forced air cooling next time.

I *AM* concerned about V5 though. Some seriously insane voltages across it. The amp sounds great but it's a worry. Would definitely be keen for some feedback on this one as exceeding the design limit by 50% is a bit out there (we're talking > 450V anode-cathode, yikes!!!).

Here are the pre-amp tube voltages as measured earlier this arvo:

Screen Shot 2011-08-14 at 10.20.16 PM.png (12.58 KiB) Viewed 1298 times

Plenty of photos and more info to follow soon.

Let me know if anyone wants me to check any other voltages whilst it's open and on the workbench...

Now off to sleep and dream of building a 400W slave to go with it

[riba2233]

Check this.

viewtopic.php?f=5&t=1311&start=30#p11564

As you can see from the thermal pictures, V5 is really hot, hotter than the other preamp tubes. But don't worry, it worked good by now, so it will work without problems

[synthi]

wow, man very impressive! one of these days I'd really like to build myself one like that!

have you measured the power output yet? I think mine will still creep up a bit further, would love to see 360 clean (120W per pair).

I read your build thread a while back but haven't gone over the updates in detail yet - are you using a 12AT7 as V5? I have 2 NOS Mullard CV4024. If one of those would work well as the driver, I may put them both in as V4 and V5. With those kind of voltages in there, I am a bit afraid of putting a modern production tube in V5.

[riba2233]

wow, man very impressive! one of these days I'd really like to build myself one like that!

have you measured the power output yet? I think mine will still creep up a bit further, would love to see 360 clean (120W per pair).

I read your build thread a while back but haven't gone over the updates in detail yet - are you using a 12AT7 as V5? I have 2 NOS Mullard CV4024. If one of those would work well as the driver, I may put them both in as V4 and V5. With those kind of voltages in there, I am a bit afraid of putting a modern production tube in V5.

Yep, I'm using ECC81 in both V4 and V5 position, Philips and Miniwatt. I think that they are tougher than ECC83

Unfortunately, I don''t have equipment to measure power output, but it's more than enough for me