Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jbefumo on February 13, 2016, 09:06:36 am
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Hope posting to this old thread isn't a faux pas, but seems to fit ...
Have my board all built for an implementation of the Tweed OD circuit. It's going to live in a Super Reverb chassis, which I expect to be delivered on Monday. Will be using some existing iron from an old Sundown SD1012, with a pair of KT88s in the output section. Will be using a tube rectifier to tame the original 100W output a bit, fixed bias, and probably a pentode/Triode switch. Also going to add reverb, and hence this question (actually, 2 questions).
First, since I have 9-pin holes in the chassis to accommodate two extra bottles from the basic circuit, I'm considering two approaches:
1) The safe, well-understood topology lifted from umpteen Fender designs -- a single paralleled 12AT7 driving, half of a 12AX7 recovering, and the other half mixing. Easy. Predictable, known to sound great.
2) Use a 6BM8 pentode/triode, with the pentode driving and the triode recovering, and then use a 12AY7 or 12AX7 configured as a common plate load mixer (see attached). I understand how it works, but am unclear on the advantages/disadvantages compared to the usual Fender-style mixer. Will try some simulations, but nothing like real advice from real people ...
Second question, which I believe I've asked elsewhere, but will reiterate here for the sake of coherence, regards the location of the reverb feed and return .
The common approach is to take the reverb signal from after one preamp stage, then mix it back with the dry signal before the next. In this overdrive scenario, I'm considering taking the feed from before the (switchable) overdrive stage, and returning it afterward. My rationale is that by so doing, a) my reverb signal will always be clean and distinct, and b) as/if the overdrive adds additional volume when engaged, as in when playing a solo, the reverb will be proportionally less than it was with the overdrive disengaged, which is something I usually want.
Actually, a semi-related third question: I'm still vacillating between fixed and cathode bias. If my assumptions are correct, I could use the 8-ohm output of the OT to drive a 16-ohm load, effectively raising the primary impedance to what's specified for those tubes in the cathode-bias configuration. I believe this alone would put my output at a more reasonable 50-60W or so, Would be a no-brainer, except that, looking at this circuit, I'm guessing that most of the tonal character is coming from the preamp anyway, so using a (defeatable) Master Volume wouldn't adversely impact the overall tone anyway, and sometimes one wants that extra clean headroom on tap --(like, I might be wanting to play bass through this rig on occasion.)
(I have a new pair of 12",8-ohm Weber Signature 12 alnicos, rated at 25W apiece, which would fit right in my existing 2X12 cabinet and should handle the lower-power scenario wired in series for 16-ohms. If I go for the higher power, I have my 'holy grail speaker -- a vintage Altec Lansing 15" 417 100W alnico, 8-ohm, which will fit the cabinet with the addition of a new baffle board.)
Thanks in advance!
Joe
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I have always been happy with a 12AX7 "one tube reverb" as I don't need surf type reverb in my amps & the 1-tube has given me plenty of reverb for what I want.
Since you are interested in having more reverb and different insertion points, then maybe you are looking at something like the attached schematic? (With higher gain amps like the TOS, I actually prefer an active FX loop over reverb & you can use a reverb and/or delay pedal)
Regarding pentode/triode switches. I think they are useful on 6V6 tubes. I don't hear much difference with 6L6 tubes? Don't know about KT88 tubes? I would think a PPIMV would be far more useful to you and just going cathode biased. Data sheet I saw looked to be using a 560 ohm cathode resistor for push/pull KT88.
I am also attaching the page on pentode reverbs that you posted in ARCHIVES.
With respect, Tubenit
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I eventually sold my original TOS. Prior to that I did add an active FX and both myself and the current owner liked it a lot. It also functions as a master volume (given the 250ka pots in the FX) and is useful in that regard also. In fact, I think one could probably do away with the PPIMV using an active FX which is exactly what I did on my current Tweed BluezMeister build. So if you are going to go fixed biased on your TOS, then the idea of an active FX might be a reasonable consideration.
I use a Digitech Hardwired Reverb in active FX loops and like it a lot. It sounds pretty similar to me to the reverb I had on my original Princeton Reverb. What I like best is using a reverb pedal and a delay pedal with both reverb and delay being subtle.
With respect, Tubenit
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Thanks!
For myself, I rarely crack the reverb over 2-3 on my Super Reverb, but would like to have more on tap, just in case. (maybe a dwell control to help tame it a bit.) Also looking toward the unlikely eventuality that I might manage to sell it. Will digest the other posts here -- much appreciated! I guess part of the motivation is always trying to do things just a little bit better -- otherwise, why bother building?
On a separate topic, just got around to opening up the 'blackfaced' super reverb (70W Ultralinear) that I bought for parts -- posted some pics "just search on 'juju'). Not only decided not to part it out, but definitely a keeper. Whoever did the conversion was an exemplary craftsman, and the job is absolutely impeccable.
I have always been happy with a 12AX7 "one tube reverb" as I don't need surf type reverb in my amps & the 1-tube has given me plenty of reverb for what I want.
Since you are interested in having more reverb and different insertion points, then maybe you are looking at something like the attached schematic? (With higher gain amps like the TOS, I actually prefer an active FX loop over reverb & you can use a reverb and/or delay pedal)
Regarding pentode/triode switches. I think they are useful on 6V6 tubes. I don't hear much difference with 6L6 tubes? Don't know about KT88 tubes? I would think a PPIMV would be far more useful to you and just going cathode biased. Data sheet I saw looked to be using a 560 ohm cathode resistor for push/pull KT88.
I am also attaching the page on pentode reverbs that you posted in ARCHIVES.
With respect, Tubenit
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I've used pentode/triode switches on EL34 amps in the past, and liked the results -- not so much as a volume/power reduction, but for adding a degree of additional tactility, for lack of a better term. Similarly, check out Dennis Kager's "RMSW control" on the old Sundown amps. They basically added a variable resistance to the cathodes of the otherwise fixed-biased power section. I think a lot of people dismissed them, expecting some kind of attenuator,but I found the debiasing effect to be, once again, more a matter of touch than perceived volume.
I have always been happy with a 12AX7 "one tube reverb" as I don't need surf type reverb in my amps & the 1-tube has given me plenty of reverb for what I want.
Since you are interested in having more reverb and different insertion points, then maybe you are looking at something like the attached schematic? (With higher gain amps like the TOS, I actually prefer an active FX loop over reverb & you can use a reverb and/or delay pedal)
Regarding pentode/triode switches. I think they are useful on 6V6 tubes. I don't hear much difference with 6L6 tubes? Don't know about KT88 tubes? I would think a PPIMV would be far more useful to you and just going cathode biased. Data sheet I saw looked to be using a 560 ohm cathode resistor for push/pull KT88.
I am also attaching the page on pentode reverbs that you posted in ARCHIVES.
With respect, Tubenit