Testing amplifiers?

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Postby Blair » Fri Jan 22, 2010 3:00 pm

Hey Mark,

That is what I'm thinking too. I'm going to start by trying to change the dropping resistor to the driver filter caps.

What does a HV regulated circuit look like? That would be a good solution if upping the first filter cap does not do it.

I tuned it because the 6550s were not supposed to exceed 500v on the plates. I had it right at 505v. When I put Kt88s in and biased it a little harder, the B+ went down to 490v, so maybe a beefier first cap is all I need.

I need to up the caps prior to the choke. I used a 5uf to tune the B+ down to below 500v. This is a dumb question, but how do you check for ripple using a scope? I've hd this scope for less than a week.

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Postby battradio » Fri Jan 22, 2010 4:38 pm

Blair ,

To measure the ripple you need a probe rated for 600 volts or geater .
Set the scope to 5 volts per devision and to AC coupling .Connect the probe to the junction of the first cap and the rectfier . The geater the load on the PS the greater the AC ripple will be .
Heres is some examples of a simple regulators for the driver stage .If you want tubes a couple of 0B2's could be used
http://www.tubecad.com/2006/11/blog0087.htm .

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Postby Blair » Fri Jan 22, 2010 4:57 pm

Thanks Mark!

I have 600v probes. I'm going to swap out those PS caps first and see if this fixes or helps the problem. I'm only looking for 60 more watts! LOL!!

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Postby battradio » Fri Jan 22, 2010 5:03 pm

Blair wrote:Thanks Mark!

I have 600v probes. I'm going to swap out those PS caps first and see if this fixes or helps the problem. I'm only looking for 60 more watts! LOL!!

Blair


You say your looking for only a Dyna MKIII !!!! worth of power ?

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Postby Blair » Fri Jan 22, 2010 5:24 pm

Sorry,

on top of the 115w I have already.

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Postby battradio » Fri Jan 22, 2010 5:34 pm

That Was A joke my smilleys don't work .
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Postby Blair » Fri Jan 22, 2010 5:41 pm

Mine don't either. It is hard to pick up sarcasm without smileys!!
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Postby EWBrown » Fri Jan 22, 2010 6:33 pm

Two "quick and dirty" tests to try:

Since the LTPI plates are clipping, and forming those "teats" on the peaks of the sine wave, the tube is getting "starved" for a proper plate voltage at higher drive levels. This is being caused by the 40V approx voltage drop in the PSU, at higher power levels. 191V on the plates seems somewhat low, and 150V on them at full drive is far too low.

Increase the value of the PSU's input cap (before the choke) it isn't a "sin" if you somewhat exceed 500VDC, try kicking it up to (no more than) 525VDC at idle (if your electrolytic caps can handle the voltage) The drop should then be less at full power vs idle conditions, Before you crank up the B+, turn the bias down on the output tubes, as the current will also rise as, the B+ rises.

The plate dissipation is the plate to cathode voltage (V P-K) multiplied by the cathode cathode current (IK), aim for about 80-85% of the max safe PD ratings.

The other Q&D test would be to parallel the 6SN7's 68K plate resistors with 220K, this would bring the effective plate load down to around 51.9K

This will increase the 6SN7's plate voltages by another 45VDC or so, and should not affect the stage's gain significantly. The CCS will keep the plate currents the same as they were with 68K. This alone may eliminate the clipping / distortion on the LTPI outputs.

I'm going strictly on memory here, from when we were working on this design last year., so I could be "off" on some of my "presumed" existing resistor values

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