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What's the worst tube amp design you've ever seen?

wa2ise

wa2ise

Super Member
Just like anything else, it's possible to do it badly. What's the worst circuit design you've ever seen in tube amps? Below I think is the worst one I've seen, likely caused by bean counters run amuck.
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Yes, the inverter is just pulled off a screen grid of the uninverted output tube of this push pull pair. :thumbsdn: I ran a simulation and it predicted that the sound would be bad. That's not the same as actually building it, and listening, but who wants to spend that time on something that will surely disappoint?
 
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Yes, screen current is a lousy function of the input voltage. Grundig used a similar circuit, but they also had negative feedback around it to clean some of the edges up... I think they also drove the cathode of the lower tube from an output transformer winding.

But my vote would be the record player amp using nothing but a 25L6, with its heater in series with the (90V) phono motor. I once owned it's solid-state offspring - a GE that had a secondary winding on the motor that powered the amp...
 
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Now that hurt, Tom. That 25L6 arrangement you described was my very first record player! As a very young boy in the earliest days of stereo, I copied the original amplifier in that unit by taking it into the electronics shop with my dad, showed the counter man all the components underneath, and asked him to put together enough parts so I could build a second amplifier. Add a stereo crystal pickup and second speaker, and voila! Thank god the pickup had separate grounds! My first venture into stereo was wonderful -- all 4" speakers and extra phono motor heater dropping "device" and all!!! Stop raining on my parade!

Dave
 
dcgillespie said:
Now that hurt, Tom. That 25L6 arrangement you described was my very first record player! As a very young boy in the earliest days of stereo, I copied the original amplifier in that unit by taking it into the electronics shop with my dad, showed the counter man all the components underneath, and asked him to put together enough parts so I could build a second amplifier. Add a stereo crystal pickup and second speaker, and voila! Thank god the pickup had separate grounds! My first venture into stereo was wonderful -- all 4" speakers and extra phono motor heater dropping "device" and all!!! Stop raining on my parade!

Dave
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My first stereo was the GE record scratcher I mentioned above, which permanently damaged many of my records... so it doesn't have any fond memories for me. FORTUNATELY, I lucked into an old Stromberg amp after college... but I had no idea where that would lead me 30-some years later!
 
As a teenager, one of my earliest amp design adventures involved a push-pull output stage using two salvaged single-ended OPTs with the primaries in series and the secondaries in parallel. For those who haven't tried it: This is a singularly bad idea.
 
Golden Tube SI50. Nice looking, but poorly built. Had all these beefy speaker and input connectors, mounted to a board that used tiny little interconnects. Worst of all was you had to use tiny push buttons to manually set bias. Unplug it, and you had to do it all again.
 
Now ya see? That's just a bad attitude, Mike. Even RCA made a buck or two off of almost the same crazy idea in all of their 171 chassis: Two single ended amps for stereo that could be connected together in push-pull for monophonic output. Besides about a zillion reasons I could think of not to do this, what's not to like about it!:)

Dave
 
As far as mechanical design goes (serviceability), I'd have to nominate the Sherwood S1000II. Two chassis bolted and wired together with a deep valley between them full of caps, resistors, and tube socket tags.:thumbsdn: I guess it makes up the trouble there with it's great sound.

The crummiest circuit design is surely one of mine, but I'm not sure which.:D
 
how 'bout dem ol' amps that used the filaments of small-signal tubes as cathode resistors for their outputs? That always struck me as emblematic of the old notion that just because you can do something doesn't mean that you should.
 
mhardy6647 said:
how 'bout dem ol' amps that used the filaments of small-signal tubes as cathode resistors for their outputs? That always struck me as emblematic of the old notion that just because you can do something doesn't mean that you should.
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Free DC bias for the most noise-sensitive tubes in the circuit? What's wrong with that?

That feature is one of the reasons the HK Award series of amps is so damn QUIET (free from noise)...

But yeah... the schematic in the first post, is about the shittiest amp design I've seen. Looks like a cheap-out when they either couldn't get, or didn't want to pay for, 6AD7s (which were like a 6F6 and a phase-inverter triode in one tube... they were made to pair up with a 6F6 to make a paraphase push-pull pair... see here: http://www.nj7p.org/Tube4.php?tube=6AD7G )...
 
mhardy6647 said:
how 'bout dem ol' amps that used the filaments of small-signal tubes as cathode resistors for their outputs? That always struck me as emblematic of the old notion that just because you can do something doesn't mean that you should.
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That sort of thing is fine, just not as implemented. It would be fine for conservative 8417's for example.
cheers,
Douglas
 
That design up top is a Philco thing. That surely had something to do with avoiding use of an RCA tube or patent or something. Nobody else was using the 41 as an output by 1941, but the only octal alternatives for the most part were things like the metal 6F6, 6V6, etc and Philco did not use metal tubes.
 
My Sparton console radio uses that screen grid inverter trick too. Admittedly it isn't the highest fi out there but it does sound pretty good for what it is. But yeah, I don't think I would build something that way.

mike
 
The hardest to repair design I've ever seen is the Fisher MPX-100. Crowded, deep and narrow under the hood. Blah.
 
mhardy6647 said:
how 'bout dem ol' amps that used the filaments of small-signal tubes as cathode resistors for their outputs? That always struck me as emblematic of the old notion that just because you can do something doesn't mean that you should.
Click to expand...

I saw this on a Fisher once, and like you, I didn't like it at all. Too much oportunity for something to go really wrong. It just doesn't make sense, especially nowadays when rectifiers are a buck.
 
mhardy6647 said:
how 'bout dem ol' amps that used the filaments of small-signal tubes as cathode resistors for their outputs? That always struck me as emblematic of the old notion that just because you can do something doesn't mean that you should.
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You guys are a little scary. I was just reviewing the schematic for a Bogen AP-30 (I have a AP-35). I'm sitting here scratching my head, why would someone use the heaters of the two 12ax7's to handle bias??? Low noise yes, smart - I dunno.

Also BTW, I have a Philco 41-290 that matches that first schematic quite well.
 
The worst tube design?

One that can KILL YOU......hot chassis.
My 1964 Harmony H400 guitar amp would have turned me into a deceased guitarist, if not for adding a small isolation transformer inside the cab.

Steve
 
By far not the worst -- but one with a couple of bad ideas, was a domestically made 300B monoblock. The factory design had a 1200 MFD cap immediately following the CV378 (GZ37). I thought that defied all generally accepted conventions for a tube rectifier. The ceramic socket for the 300B tube was fairly "ambiguous" and would allow a tube to be inserted incorrectly.

Maybe I should thank them for this design, because I picked up a pair of these monoblocks for $200, including brand new 300Bs and CV387s, 'cause the 300Bs were installed incorrectly by a local tech, who couldn't figure out what was wrong.
 
BuzzK said:
By far not the worst -- but one with a couple of bad ideas, was a domestically made 300B monoblock. The factory design had a 1200 MFD cap immediately following the CV378 (GZ37). I thought that defied all generally accepted conventions for a tube rectifier. The ceramic socket for the 300B tube was fairly "ambiguous" and would allow a tube to be inserted incorrectly.

Maybe I should thank them for this design, because I picked up a pair of these monoblocks for $200, including brand new 300Bs and CV387s, 'cause the 300Bs were installed incorrectly by a local tech, who couldn't figure out what was wrong.
Click to expand...

Most modern day 4 pin sockets suck. I've comitted to only using the machined teflon ones. They work very well, and are cheaper than NOS amphenols which run about 25 a piece.:sigh:
 
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