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So after the discussion about the 21H hookup last week, David got me thinking about how I may have really screwed up just buying another 20 watt amp haha. Please tell me if I'm crazy, which I'm sure some of you will, as to if I can convert my 21H to a 32H? I was looking at the schematics for each, and at least to me, looks almost identical! It seems that the only changes I would have to make is pop 4 6L6/5881's in there in place of the 6V6's, add a 5U4 and socket into the extra spot and wire it up, (my 21H has one 5U4 as opposed to 2 5Y3's), then change a couple resistors, and add a capacitor in. Seems like a pretty easy amp conversion to me, let me know if I'm crazy haha, as I may very well be overlooking some things.
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1949 Hammond CV w/1960 Leslie 45 (converted to 145), using H-1 and Leslie 25 amp
1958 & 63 Hammond M3
1963 Hammond L100 with 70s Leslie 120
1979 Rhodes Piano -
That seems like a great way to ruin a 21H amp. Having just serviced a Leslie 21H, I can say this amp has plenty of power when working properly. What are you trying to accomplish? Which Leslie are you using, a 145? Find a 147 amplifier, and a dedicated B+ power supply for your CV and do this right. -
And, going from 20 watts to 30 is only a 1.76 dB increase in output, which isn't that much 8) If I recall correctly, it takes a change of 1 dB for a human to notice any difference at all.Tom in Tulsa
Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720Comment
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Not necessarily crazy, but lacking critical information. The 32H amp has a considerably larger power transformer and a larger choke, and that makes all the difference.Originally posted by Alek Z View Post
6L6s draw twice the heater current of 6V6s, so if you tried to get more heater current AND more B+ current from a 21H's power transformer, you would likely burn up the 21H's power transformer fairly quickly. I have a friend with a 1960s Fender Princeton, and he burned up two power transformers before I took a look at it and pointed out to him that some genius had subbed 6L6s for the stock 6V6s.
But to answer tpappano's point, you do notice the extra power from the 32H amp, partly because it also puts a larger current through the woofer's field-coil. It also has more headroom overall in the power supply, allowing greater transient peaks. The 21H power supply is working near its limits most of the time. The ability of a power supply to meet heavy current demands (or not) tends to be audible in any amp.
A professor of mine in grad school once said something in class that stuck in my mind: "You need to know it now." This was in response to a student who clearly had not done the reading for that day. There's a certain amount of basic factual background information you need to have in your head before you start thinking about modifying amplifiers. All this information is out there on the web for anyone inclined to acquire it. No one is stopping you but yourself.
I'm David. 'Dave' is someone else's name.Comment
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More power means more power. That almost always means a new power transformer, not just discreet components.
So yes, you're kinda crazy. But I like that kind of crazy.
Yank the little amp and put in a big one. If power is what you want, it's easier and cheaper than ever to get all you need. I'm running a Crown D-150 in one of mine. Another that's bi-amped. Lots and lots of possibilities.
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All right thanks guys, just gonna leave it alone then, I figured there would be some problems but it seemed like an intriguing idea when I noticed how similar the circuitry is.1949 Hammond CV w/1960 Leslie 45 (converted to 145), using H-1 and Leslie 25 amp
1958 & 63 Hammond M3
1963 Hammond L100 with 70s Leslie 120
1979 Rhodes PianoComment
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The 21H and 32H amps are identical in many ways, but the 32H power transformer is huge, and Leslie even up-rated the filter choke. With one 5U4, the 21H amp is at or near the steady-state current limit of the 5U4 at idle. With two 5U4s, the 32H has the capacity to provide more current at maximum power output. It's like the same car with a bigger engine.I'm David. 'Dave' is someone else's name.Comment
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More like the same engine with a ported head and more compression. 🙂Comment
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People always forget how many supercharged and turbosupercharged cars there were in the 80s, too.
I had a discussion with a co-worker yesterday about 60s slushboxes. Today we have 9-speed automatics. Back then, 2-speeds were common. What brought it up is a car I have that came with 2.72 gears and a three-speed slushbox. It was like having a two-speed but with overdrive. If memory serves, getting on the freeway meant shifting to 2nd at 50MPH.
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One thing we haven't discussed here is how critical speaker sensitivity is when dealing with lower-powered tube amps. Some Leslie 25/125 cabinets had Jensen P12Ns, but others had fairly average Utah 12" drivers. A very sensitive speaker will be a lot louder with the same input power. One of my friends has a JBL D120F in a Deluxe Reverb, and it's pretty loud, even with 22 Watts.
That's why Leslie used field-coil woofers back in the 40s and 50s. It was a way to get maximum sound output from a 30 Watt amp.
I also think that the 25/125 amps can be optimized. They were obviously intended to be cheap to build with a low parts count. By the time the 25 amp was built, everyone knew that running beam tetrode screens at full B+ voltage wasn't a good idea. I would drop the screen voltage with Zeners as in the 122A and perhaps reconfigure the whole output stage like the 147 circuit, being careful to re-bias by choosing the appropriate cathode resistor value.I'm David. 'Dave' is someone else's name.Comment
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