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"Mullard 5-20 KT88 PP blocks!
"Mullard 5-20 KT88 PP blocks!
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Old 20th September 2015, 02:45 AM   #1011
BillEpstein is offline BillEpstein  United States
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After three years of learning, blundering, stops, starts, surgeries and finally a 3000 mile trek to the home of my pal Steve Brown, my Soloist 88s a la Steve are playing.

There were issues with the crummy quality control of Edcor over the 40 volts, not 60 of the bias supply. The outrageously high gain due to omitting NFB was cured by triode wiring the EF86 and padding the input with 275k resistors. Steve also recommended Mundorf MCap Supremes for the .47s. He also performed a few other tweaks simplifying the bias supply.

We are listening to the Saint-Saens Organ Symphony and loving the power of the bass with the room vibrating from the pedal tones but still having beautiful tone from the violins and wood winds. Hearing the sweetness of Alison Krause voice a little later shows the finesse possible from this circuit, the Hammond 1690 OPTs and the Gold Lion KT 88s.

Steve had built a pair back when the thread was current but wound up converting them, not that impressed, to 300Bs. Hearing these amps with the changes he made, Steve is seriously considering building them again, they sound that good. I would put the quality of sound up against any of the KR Audio amps, several of which I have had at home for extended periods.

They're are so many competing designs here and all with their proponents. The Soloist 88s deserve a place near the top of what the DiY'r should build in this category.
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Old 20th September 2016, 07:52 PM   #1012
ureche is offline ureche
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Hello, i know this is 1 year old, but i would like to ask if there is an schematic for this amp, that contains all the changes made to present ?

Thanks.
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Old 31st January 2017, 08:42 AM   #1013
psaitz is offline psaitz  Austria
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Quote:
Originally Posted by ureche View Post
Hello, i know this is 1 year old, but i would like to ask if there is an schematic for this amp, that contains all the changes made to present ?
I would like to ask the same question too.
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Old 4th February 2017, 11:43 PM   #1014
smbrown is offline smbrown  United States
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Help me think through this. I have built this amp (Rev 0.5 1/30/10) and modified it by removing feedback and yes, I know that introduces some bandwidth reduction and reduces the damping factor a bit, but I'm using the 1650 - 5k OPT and to me it sounds wonderful. However, that means way too much gain so I changed the EF86 to triode and balanced it so I have similar plate voltage compared to pentode - don't remember what that is, I think around 100v. But still too much overall gain. I'm thinking of changing the 12AT7 to something with lower amplification factor like 6SN7 or 12AY7. Since the grid voltage is fixed and the cathodes work into a CCS, are there any changes needed at the plate loads? I'm thinking not. And yes, of course I know, the 6SN7 needs a different socket. Thanks for your thoughts!

Last edited by smbrown; 4th February 2017 at 11:45 PM.
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Old 5th February 2017, 01:45 AM   #1015
kward is offline kward  United States
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I assume the schematic you are referencing is the one attached to post #316.

Serendipitously, the plate loads are, from the looks of things, about right if converting to a 6SN7.

Several things to look at:
  • The setting on the CCS will need a careful look. I'd try to tune it so that the 6SN7 is biased at about -4V to -5V grid with respect to cathode and about 8 mA quiescent current through the CCS, although this is somewhat subjective. Need the bias point set so there is enough voltage swing allowed on both upper and lower sides of quiescent point to allow clean drive of the KT88s.
  • The feedback network will need to be redone.
  • The high frequency tuning network (R12 and C11) will need to be redone.

So it's basically a whole new frontend design, but, should be a good performer when done.
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Old 5th February 2017, 02:18 AM   #1016
smbrown is offline smbrown  United States
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Thanks, kward. As I mentioned, there is no global feedback. I'm wondering if 8ma is enough (currently set to 7ma - 3.5ma per side). Let's say the B+ is 450v and the grid is sitting at 100. So the cathode would come in at 92v. If we do 5 ma per side, looks like -15v bias. And if I'm reading things correctly, plenty to swing the KT88's. Or am I missing something?
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Old 5th February 2017, 02:57 AM   #1017
kward is offline kward  United States
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My mistake. You did say no negative FB.

If the grid is at 100V, then the cathode should be at 104V or thereabouts. What's missing is the voltage on the plates under quiescent conditions. I assume that's about 250V, so that there is about 150V across the tube from plate to cathode. If that's the case, then there does appear to be plenty of voltage swing allowed.

Remember there are three parameters needed to bias a triode: grid voltage with respect to cathode, plate voltage with respect to cathode, and plate current.

From the info you have provided, I'm guessing the tube is biased at about -4V, 150V, and 4 mA. (with the grid sitting at 100V, the cathode sitting at 104V, and the plate sitting at approx 250V)
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Old 5th February 2017, 04:46 AM   #1018
smbrown is offline smbrown  United States
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Yeah, that sounds pretty good. So my plate R will need to be 5k
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Old 8th February 2017, 12:35 AM   #1019
mr2racer is offline mr2racer  United States
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Look at 5751. A lower gain 12AX7, mu drops from 100 to 70. And sounds better too.
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Old 8th February 2017, 12:47 AM   #1020
smbrown is offline smbrown  United States
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Thanks but I've already got a 12AT7 in there and I think it is gain of 70. I need lower than that.
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