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Old 24th June 2007, 01:12 PM   #1
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Default Class A 8417 Amp

I have a pair of Bogen MO60A 8417 amps. The circuit is the forerunner of the Quicksilver and it matches the Bogen MO100A circuit almost Identically. The differences are one resistor change. The MO60A circuit is very similar to what Ned says you should do if you want to correct some of the issues of the Quicksilver amps.

I want to convert these amps into Class A.

I plan to run 300VDC on the 8417
I plan to also try a 100 ohm resistor between Pins 3 & 4 of the 8417 to see what I get with triode mode.
I plan to run 10 to 12VDC neg voltage of the grids
I plan to use a 6SL7 in replacement of the first section of the 7247 and a 12BH7 as a replacment of the second section of the 7247.
I plan to leave the front end voltage as originally designed even though I plan to lower the voltage on the 8417.

Question is do I need the feedback loop from the secondary of the OPT to the cathode of the 1st amplification section?

The feedback loop is basically a 25v winding on the OPT secondary with one side to ground and the other to the cathode of the 1st amplification stage via a 56K resistor in parallel with a 15pf cap. In class A why would I still need this feedback?

Also if I really need to keep it could I not change the resistor with a 50K pot so I could dial it in as I see fit?

Thanks guys!
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Old 24th June 2007, 01:27 PM   #2
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Default Re: Class A 8417 Amp

Quote:
Originally posted by desperateaudio


I want to convert these amps into Class A.

I plan to run 300VDC on the 8417
I plan to also try a 100 ohm resistor between Pins 3 & 4 of the 8417 to see what I get with triode mode.
One must know the primary impedance of the output transformer to tell. A schematic would also be helpful.

Together with an UaIa-diagram of 8417 (think there are Us 250V and 330V available) and www.revintage.se/PPABAMP.xls you can do your own estimations. About triode I think you have to plot your own curves.

But I would go for 6550 or EL34 instead.
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Old 20th October 2007, 01:49 PM   #3
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Well I finished the Class A 8417 Triode Amp. It started life as a Bogen M60 8417 Amp. I have many 8417 and think when ran correctly they are great so it was not an issue for me to choose this tube. I started with the Quick Silver schematic and made the following changes. SS rectifier, input tube became half a 6SL7, driver tube became half a 7247, bia voltage is -12, output tube voltage is 300vdc, 8417 are in triode mode via 100 ohm resistor, the original feedback loop is removed, front tubes are ran with same voltages as original QS amp. New amp can also run 6550 in triode mode at 300vdc +/- 15 vdc. I use the Bogen M60 OPT's since I find them to be sleepers. The amps sound great, very full without any straining. If you got the time and the tubes give it a shot.
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Old 20th October 2007, 02:01 PM   #4
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As a Quicksilver owner I am quite interested in your modifications and wondered if you would mind sharing a schematic with your changes. I have also considered picking up several Bogan commercial amplifiers to play with.
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Old 21st October 2007, 01:23 AM   #5
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New baby makes that tough right now but it is right on with the QS schematic except for the following.

1. 1/2 a 12ax7 becomes 1/2 a 6sl7 with a 1K cathode resistor no feedback connected to the cathode. Plate load resistor becomes 220K This is specific to the PS I built which is SS rectified

2. 1/2 a 12au7 becomes 1/2 a 7247. Cathode resisitor is reduced to 22K. Plate load resistor becomes 15K. Again specific to my PS but I am wondering if the plate resistor and the cathode resistor must match. Can not remember the rule??? Can you clarify?

3. Coupling caps become .47@600vdc from .18 and the 150K grid resistors become 51K each.

4. Bia voltage is -12vdc.

5. 8417 plates are seeing 325vdc and they are coupled to the sg2 via a 100 ohm resitor and ran in triode mode. This could come up a little if need be since it is well within tolerance I just do not think I need it as of yet.

If you could let me know about the need for matching the plate and cathode resistors on the 7247 that would be great. If they do need to match I am hoping 15K on each works as the PS is not as high as the original.

So far for me the OPT's on the big Bogens have been very good. I have cut a few in half and they are built very well!!!

Thanks in advance on the resistor question.

Also I am run Sylvania's rebranded as RCA's on the 8417.
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Old 21st October 2007, 06:39 PM   #6
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Could you post the stock Bogan schematic?
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Old 21st October 2007, 10:27 PM   #7
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Here is the QS schematic I started with. I do not have the MO60 schematic but it is exactly like the Bogen MO100 amp except that amp uses 4 - 8417 tubes on the output and that schematic is out on the web somewhere I have seen it.

Any comments on the driver setup for the 7247 I spoke about having the 15K plate load resistor and a 22K cathode resistor?

Would it be better off if they both had the same value?
Attached Images
File Type: gif quicksilver full page schematic.gif (14.9 KB, 825 views)
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Old 22nd October 2007, 11:00 AM   #8
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Why would you use a 7247 instead of a 12BH7 or better yet a 6sN7?

How many mA thru the 7247?

What is your voltage feeding the front end?
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Old 22nd October 2007, 11:34 AM   #9
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I could be clearly wrong here but I would think that a trimmer would be in order to match the waveform off the plate and cathode side of the driver tube. The 15K with a trimmer may be the way to go.

Maybe someone with more experience could jump in here and answer this.
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Old 22nd October 2007, 11:50 AM   #10
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Go for two matched 15k resistors for the PI to work best. Maybe you could add a trimmer to adjust other unbalances in your amp.
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