Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jbefumo on March 05, 2016, 05:03:18 pm
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Hmm -- thought I posted this question, but now can't find it -- if this is duplicate, I apologize.
I know this is a load-line related issue, but am not intuitive enough yet to look at the graph and anticipate the sonic effect.
I started with a MV Super Reverb that had been very nicely converted to AB763 specs -- all new Mercury transformers, top quality caps, tube rectifier, etc. Speakers were wrong, cabinet was old, nasty, and way too heavy, courtesy of plywood & Chipboard construction, but the amp sounded great.
I moved the chassis into a 2X12 Twin Reverb cabinet (those 70W Super Reverbs were the same width as the older Twins). While the chassis was out I replaced the tubes with a pair of Gold Lion KT66s, installed a disengaged-by-default bypassable cross-line master volume, and put a mid control back in the Normal channel -- mainly because I thought the black plastic plugs looked crappy. While I was at it, I modded the Normal channel tone circuit to resemble a brown- Blonde era tone stack, using one of Doug's 4-terminal tapped potentiometers. (LOVE that tone stack, BTW.)
Checked my voltages, did my 'rithmetic, and set up the bias for 17.5W per tube -- about 70% of max.
Then, as an afterthought, I replaced the GZ34 with a 5U4Gb, which lowered the measured idle plate voltage from 469V to 446V. I redid the calculatons and raised the bias until the idle output power was again 17.5W.
So, first question: so long as the currents are within the acceptable ranges for plate and G2, is this an acceptable approach? My expectation is that the amp should be turning out the same power at idle, but will sag and compress more under increased load. Accurate? Other tonal impact?
Now things get a little complicated...
The amp came with a MM ToneClone OT with just the standard 2-ohm output. I could, get another OT with multiple outputs, but my first inclination was to put together a pair of 4-ohm 12s, wired in parallel.
I have on hand an inexpensive Celestion G12-65AVT -- 4-ohm, Chinese made, 65W, from a Valvestate, I think -- actually a decent sounding speaker. I went ahead and ordered a used Celestion G12T-100, also 4-ohm. I'm not normally a fan of Celestions in open-back cabs and/or Fenders, but they were cheap, and I figured it would give me a chance to live with it for a while and see what needs tweaking.
Since the second Celestion hasn't yet arrived, I just threw my all-time-favorite 12" in there -- an Altec 417-8H -- 100W, alnico, 8-ohm, freq range of 60Hz-8000Hz.
The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
Rechecked/reset the bias to compensate for the increased load (2.667 Ohms versus 2-ohms expected).
Again, I could easily replace that OT, and pair up the 8-ohm Altec with some thoughtfully-selected ceramic or alnico speaker, but as it stands now, the contribution of each speaker is not equal. Twice as much power is going to the 4-ohm Celestion than the 8-ohm Altec. I'll probably try pairing up that Altec with another 8-ohm 12 and see if that substantially alters the mojo.
While the Celestion is rated at only 65-watts, and is absorbing 2/3 of the power, I think it's still safe behind the 50W-Max output section. I might also try pairing up the Altec with the 100W Celestion when it comes.
So, is this a totally off-the-wall arrangement to leave as it is?
Going to make a few carefully regulated test recordings of various speaker combinations so I can perform a real comparison. Will post results as available.
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> amp should be turning out the same power at idle
If the amp is IDLE, it is turning out NO power.
We "bias" because if tubes idle too cold, small/soft sounds are rough (tubes not alive enough to pass small sounds smoothly); OTOH too hot and the tubes burn up.
There's no magic idle current. In high-voltage amps, the current for least small-signal roughness leads to excessive dissipation; so we always tolerate a little hoarseness at whisper. (We don't buy/build such amps to whisper.)
> reset the bias to compensate for the increased load
Since bias is set NO signal, the amp doesn't know about the audio load. In the ideal world, bias reading will not change. (In most of the real world, bias reads different every time you look, wall-variations and tube aging.)
I don't see a technical problem here.
I do wonder if it is a minor sin to pair a fine vintage Altec with some fresh-face new-made speaker hogging the power. I'd like one 417 alone (on a smaller amp-- "100W" is not the guitar rating). But maybe they complement each other in unexpected ways. (Murdoch and Hall just got married.)
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I do know what you mean -- I was already talking to Weber about a mate for that Altec in another 2X12 that I'm building around the TODS board -- had pretty much settled on a ceramic Chicago 12, but now i'm wondering how much that impedance mismatch is contributing. Would REALLY like to hear that Altec next to an EVM, or and SRO ...
(Also have a 418 15" Altec on hand, so wherever the 12" doesn't go, the 15 will with just a change of baffle board.)
I think recording a couple of simple passages, with identical settings, and just switching speakers in an out should tell how much is real and how much is mental ....
I do wonder if it is a minor sin to pair a fine vintage Altec with some fresh-face new-made speaker hogging the power. I'd like one 417 alone (on a smaller amp-- "100W" is not the guitar rating). But maybe they complement each other in unexpected ways. (Murdoch and Hall just got married.)
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The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
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The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
+1. Only downhill from here in my opinion.
Thanks
Mike
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If I've finally learned that lesson, I am a wise man indeed ;^)
Here are a few before/after pics
The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
+1. Only downhill from here in my opinion.
Thanks
Mike
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another ...
If I've finally learned that lesson, I am a wise man indeed ;^)
Here are a few before/after pics
The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
+1. Only downhill from here in my opinion.
Thanks
Mike
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Bought it for parts, and THIS is what I discovered inside:
another ...
If I've finally learned that lesson, I am a wise man indeed ;^)
Here are a few before/after pics
The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
+1. Only downhill from here in my opinion.
Thanks
Mike
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Bought it for parts, and THIS is what I discovered inside:
another ...
If I've finally learned that lesson, I am a wise man indeed ;^)
Here are a few before/after pics
The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
+1. Only downhill from here in my opinion.
Thanks
Mike
Very nice work and a whole bunch of money in iron. Great purchase.
Thanks
Mike
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I KNOW -- I really did a double take when I looked underneath.
Of course, NOW I have that 4X10 cabinet beckoning .... Thinking about a bass amp -- say ... 200W from a quad of KT-120s ....
Bought it for parts, and THIS is what I discovered inside:
another ...
If I've finally learned that lesson, I am a wise man indeed ;^)
Here are a few before/after pics
The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
+1. Only downhill from here in my opinion.
Thanks
Mike
Very nice work and a whole bunch of money in iron. Great purchase.
Thanks
Mike
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The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
but of course, THAT'S not going to happen .... Actually, I'm just really curious to see if, and if so how much, that impedance imbalance is contributing to the tone. Unfortunately, I don't have anything in 8-ohm that remotely resembles what I'd actually put in there, but should provide some indication whether it's the Holy Grail or Alzheimer's/hearing-loss...
Which got me to thinkin' ...
If I were to find an ideal match in an 8-ohm speaker, and wired them up for a net 4-ohm load, I know it won't destroy the amp, but permissible and ideal can be a wide gap.
As it stands, I had to set the bias as high (toward zero) as it would go to maintain the same power output when I changed to the 5U4GB rectifier. If I were to switch back, in order to raise the voltage back up, would it be reasonable/possible/advisable to push up the bias to compensate for pushing a 4-ohm load on a 2-ohm tap? Outputs are still on the fuzzy edges of my learning curve. My ASSumption has been that a) the G2 current is proportional to A current, so b) I can raise the Kcurrent as much as I want, so long as I never exceed the maximum stated for either the plate or the grid. Is this accurate? Truth is, if I end up going that route, I'll bite the bullet and buy a multi-tapped transformer from MM. There are several plugged holes in the back plate so I could certainly accommodate an impedance switch.
(Have my eye on a Fender/EV 12" from around the ill-reputed vintage of the original amp -- seems like it would make for some kind of cosmic balance to have in in there. Would also help balance out the weight of those oversized transformers.)
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Well, tried it out connected to my much beloved, long-horded Altec 418, also 8-ohm, and that convinced me: going to track down a second 417, and swap the OT to support 4-ohm cabinet.
-------clip-----
Not sure if for-sales are prohibited, so if this gets clipped, I understand -- just put my 418 on Ebay for a buy-now of just enough to cover buying the aforementioned 417, so if anyone is remotely interested, have it at a price point where I wouldn't consider selling it, were it not to replace with something similar:
http://www.ebay.com/itm/-/222044846031? (http://www.ebay.com/itm/-/222044846031?)
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417 is a speech-range LOUDspeaker.
418 is the same but tuned for BASS. More mass, more excursion. If you played an octave lower than guitar, and had a large box, the added deep-bass of the 418 would show itself.
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418 has a frequency response of 45Hz to 8000 Hz -- it's definitely touted as a guitar speaker. There was another model in that era (421, perhaps) that was a proper bass speaker. This is more along the lines of a JBL D-130.
417 is a speech-range LOUDspeaker.
418 is the same but tuned for BASS. More mass, more excursion. If you played an octave lower than guitar, and had a large box, the added deep-bass of the 418 would show itself.
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Not sure if for-sales are prohibited, ........
If you scroll down on the main forum index page at the very bottom is Buy - Sell - Trade, link below;
http://el34world.com/Forum/index.php?board=3.0 (http://el34world.com/Forum/index.php?board=3.0)
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Thanks!
Joe
Not sure if for-sales are prohibited, ........
If you scroll down on the main forum index page at the very bottom is Buy - Sell - Trade, link below;
http://el34world.com/Forum/index.php?board=3.0 (http://el34world.com/Forum/index.php?board=3.0)
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:icon_biggrin:
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So, at the risk of belaboring this issue to death ...
Have gone though a bunch of speaker combinations -- matched impedance, mismatched impedance, including the Altec, excluding the Altec -- recording the same 2 simple passages at the same 2 gain levels.
Pretty certain the Altec is the source of what I'm hearing and liking.
Waiting now for two 417As to arrive (too good a deal to pass up, and the 417A is, if I'm not mistaken, a bit less power-capable (75W vs 100W) then the 417-8C.) Both are 8-ohms. So to get a bit of a leg-up, I hooked up the old Altec 417-8C in parallel with an old 8-ohm alnico Rola 12", just to get a sense of whether it would bias properly with the increased load.
Seems to be fine, and sounds good, though I can't really push it much with that 15W speaker in the mix. Biased to ~38mA/Tube at 469V Plate voltage, which, if I'm ciphering correctly, should be giving me 35.64W with no signal. So, if I call that 70% that puts my max at 50.92W peak.
Back again to the original question, or a variation thereof: By my calculations, the amp should be turning out full power, even though it's pushing twice the load the OT is purportedly designed for, correct? This being the case, is there ANY benefit in replacing that OT with a similar MM model with a 4-ohm tap, or will it make no difference whatsoever?
(The only reason I thought of this was that I stumbled across an assertion that the JTM-45, with a pair of KT-66s, used a 6k load, and in fact, although the Super Reverb transformer is supposed to be presenting a 4.2K load, 6L6s have virtually the same specs as the KT66.)
At this point, I'd have no compunctions about biting the bullet and replacing that transformer, even if for just an incremental improvement (and to be able to say everything is just so), but don't care to do so if it's an utter exercise in futility.
Thanks again,
Joe
The problem is, it is now hands-down the best sounding Fender of any model or generation I've ever played.
I'd say you're done. Wish I could run across that problem occasionally. :icon_biggrin:
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... I hooked up the old Altec 417-8C in parallel with an old 8-ohm alnico Rola 12", just to get a sense of whether it would bias properly with the increased load.
(Oh yes, I pulled the 5U4Gb and put back the GZ34 to get the voltage back to where it was.)