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    Building, troubleshooting and testing of these amplifiers should only be
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Sherwood S8000 transformers

mr2racer

Member
2009-07-09 12:53 am
Hello All,

Just bought a pair of Sherwood S8000 OPTs. I know they were meant for 7868 tubes in a PP configuration. Also that 7591's could be substituted. My question is this, since I only have the transformers at this point I'm free to build any circuit that uses compatible tubes, 7868,7591, 6l6... so any ideas what would be the best sounding amp to copy?

And does anyone know where I could get an electronic copy of the Sherwood schematic?

Kevin
 
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mr2racer

Member
2009-07-09 12:53 am
Eli,

Is there any reason not to "trust" the iron? The transformers don't look scorched at all. And they were taken from a working amp. At least that's what I'm told. If they ohm out OK can I expect them to work? Is there a better way to test them?

I'll look into the HK. I had an A-401 when I was in high school. I sold it to a friend in 1980. And I've been trying to buy it back since. Its hard for me to let things go.

Kevin
 

mr2racer

Member
2009-07-09 12:53 am
Eli,

I see what you mean. The transformers in that picture are roughly twice the size of the Sherwood's. What about a copy of the Sherwood amp? Or, I believe Fisher used the 7868 and the 7591 tubes also. Are there any other circuits you could recommend?

Kevin
 

mr2racer

Member
2009-07-09 12:53 am
Hi Eli,

If you are still online. I just recieved the transformers. You were right they appear to be quite small. I haven't been able to find a schematic for the S8000 but I do have an S5000. Since they both use 7868 tubes is seems likely that they have the same amplifier sections? I saw a spec sheet for the s8000 that said 65 watts for both channels. That seems likely to be one of those old "peak power" type specs.The specifications for the S5000 claim 20 watts per channel. These transformers seem more consistent with a 20 watt amp.

Also, I looked at the pinouts for the 7591. Those two parallel elements connected to pin 5, is that a grid?

Kevin
 
Kevin,

Look at Tungsol's 7591 data sheet. 30 WPC RMS is easy enough out of a PP pair. So, that claimed 65 W. total could be quite honest.

Pin 5 is a common connection for the cathode and the suppressor grid (g3).

Notice that the screen grid (g2) is connected to both pin 4 and pin 8. Take no prisoners. Mount a 470 Ω Carbon comp. stopper/current limiter at each of those pins and tie the 2 resistors together to form the g2 connection point.

Fact, 7591s are as easy to drive as EL84s and 6V6s. That means you can build an "El Cheapo Grande" using the EC's 12AT7 based splitter/driver. The key facts are 200 to 220 V. on the 'T7 anode and an IB of 3 mA. I leave it as an exercise for you to work out the parts changes needed.
 

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Seeing as you have the transformers, build an amp with them and see what you get. I built my first P-P amp using iron from a Fisher 500B receiver and EH 7591A outputs (totally different circuit, though - check the "Kingfisher thread). The Fisher transformers aren't as big as the HKs (still not all that skimpy, though), but the sound is awesome, even with speakers that are power hogs. I've never bothered to do any formal measurements, but the power is probably 25-30W/channel.This is still quite enough power to make some serious noise, unless you've been listening to metal at full volume for a few years....

Edit - send us a pic of the iron so we can compare to the Fisher 500B and 500C outputs
 
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mr2racer

Member
2009-07-09 12:53 am
Hey All,

I have a pair of Sherwood S5000/S8000 output transformers. They originally used 7868 output tubes. The preamp and phase splitters were 12AX7's. The Dynaco 120, a 12 watt mono design in the Dynaco transformer catalog used one 12AX7 and two EL84's. I'm planning to build some of these amps and have some 6P15P's ordered, but they will take a couple weeks to get here (from Russia). I'm planning to use two 6GK5 triodes instead of the 12AX7as driver/phase splitters. (On Eli's advice, almost as much mu and more amperage than the 12AX7).

This is my question, as in most amp designs, the two halves of the amp divide along the line drawn across the coupling caps. Is there any reason I couldn't use the 6GK5 front end from the Dynaco design and the Sherwood output section and transformers? This would give me a 32 watt stereo amp with vintage transformers?

Also, my choice of tubes seems to be EH 7591a, JJ7591s, EH 7868, or NOS tubes. Any recommendations?

Kevin
 
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Kevin,


A good thing about 7591 family tubes is the fact that small signal circuitry designed for 6V6s and EL84s requires no changes. Fisher and Scott took full advantage of that fact. You can too. If the O/P trafos lack UL taps, regulate screen grid B+.

As for the O/P tubes. JJ's Octal production is junk. NOS is VERY scarce, expensive too. EH 7868s cost more than EH7591As. So, the EH 7591A gets the nod.
 

mr2racer

Member
2009-07-09 12:53 am
Hi Eli,

I would plan on running B+ around 400 volts. When you say regulate screen grid B+ are you talking about connecting the grid to the plate via a 100ohm resistor? And since the screen only contributes a couple watts could you leave it unconnected? Or does it need to be connected to dissipate energy?

Lastly, is there any difference in sonic quality between the EH 7868 and the 7591A's? If there isn't one that is sonically superior. The lower cost and octal socket of the 7591 would argue for that tube. Thanks as always, Kevin
 
Screen grids have to be connected, for proper function. A 100 Ω resistor between screen and plate is triode mode. That's simple and works well, but about 15 W. from a PP 7591 pair is the yield. To get "30" W., either UL or full pentode mode must be employed. In full pentode mode, g2 is directly connected to a positive voltage. Good results from full pentode mode requires electronic regulation of g2 B+ stability. The Maida Regulator shows how it's done, not at all simple.

Just use the EH 7591A. For monetary reasons, I would expect New Sensor to use the same "guts" in both the 7591 and 7868.

BTW, examine the 7591 data sheet. Lots of good info. in that document.