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Old 15th September 2006, 01:37 AM   #1
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Default junkbox amp help

Thanks to mwiebe I've determined I have a Tektronix PT that should be useable in my next amp.

Secondaries are:

520VCT/250ma
430VCT/150ma
6.3VAC 12.5A
6.3VAC 12.5A
5VAC, 4A
5VAC, 2A

What's I'd like to do is something like Triodedicks MonoBill amp in a stereo form, but the PT is not even similar to the specified.

I have a pair ST70 OPT that I'll use.

I've messed around with PSUDII a little using tv 6au4GTB damper diodes (and a few others), and the st 70 choke, trying to get in Dick's B+ ranges, but frankly I'm a noob so I have no idea if I'm on the right track. So far I'm always coming up with warnings of IFRM. Considering all of the 6.3V excess, I've set up a bridge rect using 6AU4's to CLCRC in an attempt to get the voltages to drop enough but I always get IFRM warning. Of course I'm not sure if I calculated the load correctly either. I came up with approx load of 11K(???)

Anyone with some pointers? I'd like to get this going without investing in new iron or chokes.

TIA
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Old 15th September 2006, 02:40 AM   #2
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Reduce the input cap to reduce IRFM (peak repetitive forward current).
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Old 15th September 2006, 07:20 AM   #3
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in psudII, try to find out the current requirement, much easier than using resistive load
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Old 15th September 2006, 03:28 PM   #4
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Well I believe the current should be 4x KT88 or eL34 @ 50mA each, plus two ECC99 at 15mA to 20mA each. I'm thinking of only running the power tubes off of the 520V/250mA winding and operating the ECC99's with the 454V/150mA winding. So for the power tube PS I have entered a 200mA current sink....sound OK?

Power tubes alone:
With a CLCRC I've come up with: 6AU4 Bridge-1uF-1.5H-500uF-20R-200uF, the 400V RMS is too low (assuming this is the B+)

With LCRC of same bridge-1.5H-10uF-20R-500uF, the RMS is higher at 412V, but still too low. Plus, there is a PIV warning which I don't understand considering the diodes PIV rating is 4500V!
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Old 15th September 2006, 04:43 PM   #5
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Random thoughts..

The amount of overall capacitance and the way it is distributed seems excessive.

Why not aim for something closer to 500V? More power and better linearity.

Four 6AU4 in a bridge are fine, but you can do the same thing with a single 5AR4 and a pair of silicon diodes.

The ST-70 choke is not wonderful, cheap surplus chokes are readily available on ePay and at local hamfests. Hammond makes cheap chokes. Say something with a dcr of 100 ohms or less and 5 - 10H inductance rated at 250mA or greater.

I know you don't want to spend the money, but considering the effort you are going to put into building this amp why scrimp by reusing what has to be one of the most marginal chokes ever designed?

Here's a can of worms - have you considered choke input with the bridge rectifier?

I'd use 50 - 100uF per section and I'd also dispense with the 20 ohm/200uF thing altogether. Solen films work great and are pretty inexpensive when purchased from Solen in Quebec.

In PSUD you need to add the choke's series resistance to the simulation, and a small amount of resistance in series with each el cap. I have had some inconsistent results, usually due to "user error" but not always.
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Old 15th September 2006, 08:20 PM   #6
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Thanks Kevin.

I'm sure you're right regarding the choke but at this point I want to see what I can get with what I have. The choke is 1.5H, 55R, 200mA, 2500VDC highpot tested from Triode Elec, hopefully better than the original.

I've tried choke input followed by 250uF but still have 3V of ripple with ~453VRMS, max is 481V. (Is RMS the B+??)

Is 3V of ripple or more acceptable?

With less C the ripple goes way up. Using L-C-R-C where C=47uF and R=50R the ripple is ~5V with 460V RMS. I suppose if 5V of ripple is fine I'll try to put this thing together. Using 100uF instead gets the ripple to 2V or so.
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