One thing I noted last night during a bout of extended operation was that the power transformer was getting warm (somewhere between 45C and 50C). Speaking for the novice, for a metal surface with high heat capacity, 50C is ~10 seconds uncomfortable, meaning you can leave your finger on for ~10 seconds before really needing to pull it off. 55C is ~5 seconds uncomfortable, and 60C is ~1 second uncomfortable. Such is the nature of the non-linear thermometer built in to our sense of touch to protect us from our own insatiable curiosity. Glass and plastic surfaces with lower heat capacity have higher "uncomfortable" temperatures. This realization is actually acknowledged in UL specifications for maximum allowable surface temperature.
Anyway, my power transformer is getting warm. A temperature rise of 25-35C above ambient is actually very conservative design for a power transformer. If an engineer did that these days, the bean counters would no doubt be all over him for wasting money on excess copper.
However, my spider-sense is tingling, so one of the things I'll do when the amp is down in the basement for mods is to add the top panel tip jacks for monitoring output stage bias current that I meant to have all along, just so I can keep watch to make certain that the bias current in the output stage isn't slowly running away with me. Probably not, but it never hurts to check. Even if there are no problems, the top panel jacks will come in useful when it comes time to load in another set of output tubes.
Anyway, my power transformer is getting warm. A temperature rise of 25-35C above ambient is actually very conservative design for a power transformer. If an engineer did that these days, the bean counters would no doubt be all over him for wasting money on excess copper.
However, my spider-sense is tingling, so one of the things I'll do when the amp is down in the basement for mods is to add the top panel tip jacks for monitoring output stage bias current that I meant to have all along, just so I can keep watch to make certain that the bias current in the output stage isn't slowly running away with me. Probably not, but it never hurts to check. Even if there are no problems, the top panel jacks will come in useful when it comes time to load in another set of output tubes.
Interesting thread. I have an old 800B in storage that will eventually be rejuvenated as a bedroom receiver. My original thinking was to use a 7591 variant of SY's RLD amp and Morgan Jones RCRC filtering. I need to build a new timber case so I can extend it to add room for all the extra caps, LEDs and screen regs to keep it in a condition that I can revert it to stock later if desired.
Say, what's the specs on the 500B power transformer? I've got one here, code T860-142A. Haven't wired it up to check but from the user manual schem it's got 2x6VAC, i'm guessing a 12VAC and what is the high tap, 200VAC? Any idea of current for these taps?
The 500B power iron has one HV winding with no taps, meant to be run into a voltage doubler, a a pair of 6.3V windings, and a bias winding - that's it. The schematic for the 500B is widely available on the web. From that you can look at the tubes hanging off of each of the 6V windings to estimate capacity.
Hello everyone. I have a group of parts that I was hoping to build an amp with. Power transformer is from a Fisher 800b. I have four EH 7591A's. And two Acrosound TO300 output transformers. I'm not sure if these will really work together. The reason is that every 7591A amp I've seen is a "Full Pentode" where the Acrosounds are ultralinear. Can the 7591A be used in an ultralinear circuit? or should I just buy a set of 6l6's and keep the 7591a's for another project?
And yes I now have a copy of Morgan Jones and am reading it.
And yes I now have a copy of Morgan Jones and am reading it.
A circuit employing 7591 "finals" wired for UL mode is fine. 😀 Check the TungSol 7591 data sheet for operating conditions.
A word to the wise, EH 7591s are intolerant of liberties being taken with the grid to ground resistance limit. Any number of "vintage" designs, ie. Fisher, took liberties there. Don't copy those designs.
A word to the wise, EH 7591s are intolerant of liberties being taken with the grid to ground resistance limit. Any number of "vintage" designs, ie. Fisher, took liberties there. Don't copy those designs.
Hey All,
Attached is a schematic of the TO300 circuit published in the Acrosound catalog as modified by Keith Snook. I want to build the amp using a Fisher 800b power transformer and a SS power supply with regulators. Once I get the voltage after the choke I can do the power supply myself. Instead of the 6L6's I want to use EH 7591A's. I know I need to swap the pin 8 connection to pin 5 and pin 5 to pin 6. Then I think I have to connect pin 4 to pin 8.
I want to change to fixed bias and the voltages from the Tung-sol data sheet Eli turned me on to for the output tubes.
What I need to know is the arrangement of the pot/cathode resistor/bypass cap. Any help would be much appreciated.
Kevin
Attached is a schematic of the TO300 circuit published in the Acrosound catalog as modified by Keith Snook. I want to build the amp using a Fisher 800b power transformer and a SS power supply with regulators. Once I get the voltage after the choke I can do the power supply myself. Instead of the 6L6's I want to use EH 7591A's. I know I need to swap the pin 8 connection to pin 5 and pin 5 to pin 6. Then I think I have to connect pin 4 to pin 8.
I want to change to fixed bias and the voltages from the Tung-sol data sheet Eli turned me on to for the output tubes.
What I need to know is the arrangement of the pot/cathode resistor/bypass cap. Any help would be much appreciated.
Kevin
More information,
The original Acrosound schematic lists 400 volts out of the power supply. And Tung-Sol recommends 400 volts on the plate with -20.5 volts for the bias supply. So 400 volts seems like a good place to start.
Kevin
The original Acrosound schematic lists 400 volts out of the power supply. And Tung-Sol recommends 400 volts on the plate with -20.5 volts for the bias supply. So 400 volts seems like a good place to start.
Kevin
If you want to do this, why not start a separate thread instead of trying to tag on to this one?
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