Capacitor or Choke Input?

Never really dug into it too much, but an interesting discussion going on one of the other forums. Are the Dynaco clones considered capacitor or choke input? Or more of a hybrid?

The question concerns rectifier ratings. I've been under the assumption that the Mullard GZ37 I'm using tops out at 350mV current, but digging into the spec sheet, it looks like that's for choke input. Capacitor input tops out at 250mV.

Then again, I've seen comments that the tube is highly underrated, and should actually be capable of around double that at least.

Here's a link to the spec sheet ...
http://www.r-type.org/pdfs/gz37.pdf

My take on it ...

Loosely interpreted, the Dynaco circuit is capacitance input. A capacitance input "pulses" the rectifier, which is what stresses it and makes for fireworks. Dynaco's design minimizes capacitance yet compensates for that with larger chokes. Where chokes are primarily there to filter the AC component out of the DC, in this case, they do double duty to smooth out the pulses, reducing the stress on the rectifier itself. The ST120 does increase capacitance, but well within what one would think of as reasonable limits to allow the higher output while staying within the design.

I've seen comments that the GZ37, "properly choked", should be able to handle up to around 700mA current. I interpret that to read the tube should be able to take anything any of the Latino amps can throw at it, even with a quad of KT120's at 60mV per tube ...

I have Been running my Mk3 Without a Choke...An I must say thay sound better than ever...so Don't dare pull the choke OUT, are you wont put it back! Also all my tube amps have Bob Carvers  DC restorer Setup on the output tubes.....this has added to the great sound an works great with the choke out. Carvers DC restorer Mod is the best tube MOD I have ever seen in 40 years of owning tubes, hower I have not put this mod on any of the h/k Citation amps I owned in the past...I never cared for the sound of the  Citation amps...others realy like the sound....
I run the Mk3s with the old  GZ34  rectifiers....An the sound of diff in the rectifiers are vary ezey to hear....with the choke in ...not so much....there eather Bad are Good...sounding...
all just one tube nuts finding...........




thanks Bob........................

Good thread. What say you Bob?

Good links ... lot to soak up.

Consensus so far seems to be that the GZ37, like most any bottle, will be working at the high end of spec for operational limits, BUT ... with proper care and setup, should be pretty durable. Way more so than a Chinese 5AR4 anyway. Mine's done yeoman duty for several months now, so I'm a fan.

And speaking of stressing the rectifier ... I've also done the "yellow sheet" diode mod, and drop the wall current (124+ average) to around 117vac using a bucker. I've also got a thermister on the AC line that slows inrush and also drops the wall current another point or two as they never go fully open. In any case, I think I get bonus points for trying to give the GZ37 it's best shot at a long and healthy life in this amp.

PS ... you may remember my experimenting with a Phillips 5R4GYS as a fairly inexpensive alternative to the GZ37 ... that didn't go well at all. Tube lasted about a month in this amp. I've seen reports of good results from others, so I may just have gotten a bad tube.

zx wrote: Also all my tube amps have Bob Carvers DC restorer Setup on the output tubes.....this has added to the great sound an works great with the choke out. Carvers DC restorer Mod is the best tube MOD I have ever seen in 40 years of owning tubes

Don't believe I've ever run across that configured for a KT88 based amp. Got a schematic?

sKiZo wrote:Good links ... lot to soak up.

Consensus so far seems to be that the GZ37, like most any bottle, will be working at the high end of spec for operational limits, BUT ... with proper care and setup, should be pretty durable. Way more so than a Chinese 5AR4 anyway. Mine's done yeoman duty for several months now, so I'm a fan.

And speaking of stressing the rectifier ... I've also done the "yellow sheet" diode mod, and drop the wall current (124+ average) to around 117vac using a bucker. I've also got a thermister on the AC line that slows inrush and also drops the wall current another point or two as they never go fully open. In any case, I think I get bonus points for trying to give the GZ37 it's best shot at a long and healthy life in this amp.

PS ... you may remember my experimenting with a Phillips 5R4GYS as a fairly inexpensive alternative to the GZ37 ... that didn't go well at all. Tube lasted about a month in this amp. I've seen reports of good results from others, so I may just have gotten a bad tube.

zx wrote: Also all my tube amps have Bob Carvers  DC restorer Setup on the output tubes.....this has added to the great sound an works great with the choke out. Carvers DC restorer Mod is the best tube MOD I have ever seen in 40 years of owning tubes

Don't believe I've ever run across that configured for a KT88 based amp. Got a schematic?

another option, providing you got the room and mounting hole, is to parallel up on two rectifiers, each will only work at about 50% capacity or so.......

Easy enough ... on paper anyway.



(Gotta love FPE's software)

Bit more work on the real thing ... maybe next time ...

sKiZo wrote:Easy enough ... on paper anyway.



(Gotta love FPE's software)

Bit more work on the real thing ... maybe next time ...

You might want to spread those rectifiers out a bit more. They get hot!

Is there a link or a schematic for the Carver DC restorer mod or any details, info on it ?

Got on it myself and found this bit of info on Audioasylum.

Hey Skizo:
It covers the KT88 circuit so it seems to be a viable option for that circuit too. check it out:

The DC Restorer
Carver says he copied the DC restorer from old TV circuits with tube video amplifiers. "Those sets needed to deliver the DC component of the video signal to the CRT all the way down to DC. As used in my amp, this circuit reduces distortion by a factor of three as well as the tube dissipation by approximately the same amount. It works by keeping the DC component on the control grids the exact correct value over the whole signal swing, getting rid of the need to operate the tubes at very high currents (even at low level operating conditions just to get them to work right). The main thing that causes non-linearity in tube amps is that the screen voltage drops when the amp is driven, not so much because of low idle current to begin with (though higher idle current does make for lower distortion, but the trade is a bad one). It takes lots of idle current to drop the distortion a substantial amount, so the DC restorer is a better choice than lots of idle current. I get lower distortion at eight watts idle per tube than at 28 watts idle per tube."
The restorer is simple yet ingenious. It uses a 6AL5 dual diode. Each diode section services one bank of power tubes. The 6AL5 cathodes are connected to the control grids of the push-pull power tubes which are held at a nominal fixed bias of -47VDC. The diode plates are at an acquisition threshold of -56VDC. Bob admits that some aspects of the DC restorer operation are somewhat mysterious to him, as they are to me as well. My take is that the circuit aids significantly in recovery from hard cutoff conditions. Under those conditions the KT88 grids act as rectifiers and shift the effective DC level below -56V, to the point of causing the 6AL5 to conduct momentarily and pull the DC bias back to its nominal value. Bob estimates the lifetime of the 6AL5 as about 50 years. "I know that seems wrong, but 6AL5's are ubiquitous in tuners, and 50-year old tuners always have these tubes and they still check as new. There are so many of them in this world that a guy on eBay sells a string of them for three cents (each tube) to be used as Christmas tree lights."

Another interesting link that suggests you could replace the 6AL5 with a couple SS diodes ... that'd certainly make the mod a whole lot easier to stuff into even the stock chassis ...



http://www.rfcafe.com/references/popular-electronics/semiconductor-diode-may-1961-popular-electronics.htm

... or maybe even incorporate into the design if it works as well as suggested. Then again, Carver could get a bit ... mystical, as well as mystifying at times ...

"Bob admits that some aspects of the DC restorer operation are somewhat mysterious to him"

I have the DC restorer (clamp) in all my tube amps now....I only use SS diodes..
Bob has said the 6AL5 tube...Is a little sweeter sounding but the SS diodes work fine..
One thing I do diff from Bob setup... is I put the diodes at the grids of the output tubes ...not at the front of 1k res. that feed the grids....this sounds better to me....
I have put this mod on some  VTL Deluxe 140 mono 807 amps...The owner thinks it a night an day diff in sound...for the better ...like a new amps...
this clamp keeps any of the output tube from being pulled down when you start to drive the tubes hard....It gives a more powerful an sweeter sound.... as it should when the output work closer togather....No matter how good the Matchet output tubes are ..
.NO transfourmers two winding are perfice....one side always get pulled down.....not with the DC restorer (clamp)...
this give a sound  that I have never got out of any tube amp...an what 2ea 5cent diodes an a res. an a little time....
I have one pr of my MK3s running with out a Choke on the output tubes...the Choke is only on the frontend 6AN8....With the Clamp setup in.. I can run two diff make of output tubes...hehe.. it wild I have one GE 6550 an a Sovtek 6550.. in ea amp...Is sound like To GEs with More output..Also I have been running a KT66 an a EL34 with vary good sound...never any tube run away...
Don't do this with out the Clamp!.....
.Vary Cool mod.. An I see NOT any Down side..
.All the Amps I have put this Clamp in work better an the sound...well if you don't go you wont Know...An just when I thought I had saw an done it all with tubes...just saying
Have fun with tube sound
Thank's To Bob Carver for giving it to us all...




thanks for this site Bob................

This is a great thread and the topic seems worthwhile to explore more in depth.
We are getting a lot of dialogue to describe this mod but little in the way of actual schematics from those who have used it and I for one would like to see a few visuals of the various ways to do this.

I would like to hear in particular from the users who have it installed in Dynaco/VTA amps and any more observations about the pros and cons between diodes and tubes.This mod sounds like something we can actually hear and enjoy.
If anyone out there has a schematic please post it because I for one am contemplating testing it in a VTA amp and want to have as much input on the best setup.
Skizo makes a good point about the diodes versus tubes being easier to fit in the original chassis but what if any are the downstream consequences to the rest of the circuit? For example does it affect the ideal bias setting in any way?
Bob/Roy: Since you two are the most knowledgeable and most experienced at tinkering with our favorite circuit do you have any suggestions you can provide in the form of a diagram or other advice on this mod?

corndog71 wrote:

sKiZo wrote:Easy enough ... on paper anyway.



(Gotta love FPE's software)

Bit more work on the real thing ... maybe next time ...

You might want to spread those rectifiers out a bit more.  They get hot!

actually I got mine pretty close, the highest temp I recorded with my IR temp gun on the rectifiers is 195F...which considering is not that bad at all!

Look Don't fear Bob Carver....Mr Pass of Pass labs these guys are Vary helpful....E-mail Bob Carver ask him any thing about his  DC restorer (clamp) mod.... are any other thing About tubes..it may take some time but he,ll get back to you..
But there more info on the Craver site on the DC restorer (clamp)
on the Audioasylum. site theres h/k Citation 2 schematic that shows the MOD...I put that same ss diodes  steup in my MK3s over two year ago.....I well never run a tube amp with out this mod!
But that's just me.....one more thing about the MOD the power transf runs a lot cooler with this setp even when the tube are bias up here....
http://www.bobcarvercorp.com/#!contact/csxp




thanks for the site Bob......

baddog1946 wrote:This is a great thread and the topic seems worthwhile to explore more in depth.
We are getting a lot of dialogue to describe this mod but little in the way of actual schematics from those who have used it and I for one would like to see a few visuals of the various ways to do this.

I would like to hear in particular from the users who have it installed in Dynaco/VTA amps and any more observations about the pros and cons between diodes and tubes.This mod sounds like something we can actually hear and enjoy.
If anyone out there has a schematic please post it because I for one am contemplating testing it in a VTA amp and want to have as much input on the best setup.
Skizo makes a good point about the diodes versus tubes being easier to fit in the original chassis but what if any are the downstream consequences to the rest of the circuit? For example does it affect the ideal bias setting in any way?
Bob/Roy: Since you two are the most knowledgeable and most experienced at tinkering with our favorite circuit do you have any suggestions you can provide in the form of a diagram or other advice on this mod?

Agree. This is all just a tease! Show me how is done...

Pedro, your not kidding. This is just the biggest tease.  I so want to hear this!!!  THis may help:

Bob Carver also did his clamp diode modification trick and re-biased the KT-120s to 26mA idol current

Citation_II_mod_schematic_001.jpg

You can install the DC restorer by using a pair of 4148 diodes connected anode-to-anode, the cathode of each one connected to the grid of each output tube. Connect the common anode connection to the lowest (most negative) bias voltage(-55 VDC)through a 91K, 1/2 watt resistor. In other words, one end of the resistor goes to -55 volts, the other goes to the common anode connection of the 4148's. Set the idle current for 26 ma on each output tube. To obtain enough range with the existing controls, you will need to remove R30 and R69 (Sam's schematic), the optional 18K resistor shown on the schematic. Some factory-built units already have it removed, so check your unit to see how it came from the factory so long ago. A vacuum tube 6AL5 tube will work slightly better than the 4148, as it's smooth turn on characteristic does not induce the small non-linearities that the 4148 does."

DC restorer ...works! ....an No Bob Carver was not the first one to use this....been around for over 50 years....older TV would not have worket with out this.....Bob did use it in his tube amps... ..anyone can do this mod... it vary ezey..... on any tube amp too see-hear.... for them self..
what is the so called tease?? .....hehe it funny....
cost is what less then $5......
Some say it make the sound better....what not to like?.....good luck have fun with tubes....





thanks for the site Bob....................

Calling all Sub-Genius EE or even Genius EE. Really, if anyone out there in cyber land has figured out how to apply the DC-Restorer circuit to a Dynaco MK III/VTA MK III and is willing to share their findings I certainly would like to see that schematic!

seems this thread has turned into a DC restorer thread, rather than cap or choke input thread
The DC restorer thread was done last summer, here is a link to the mod done on an HK Citation2,
should be near identical for any Dynaco push-pull amp . . .
http://www.vacuum-tube.eu/images/hk_citation2_mods/hk_citation2_mod2.jpg
somebody get two diodes and a resistor and try this . . . and let us know . . . (other than ZX who has done it)
Sorry, I don't have a tube amp handy, everytime I build one someone buys it !!