[guest1632]

Hi all, Well, for now, I'm gonna get this GGP. Now, just curious before I do anything to it, besides the volume pot, and Selector Switch replacements, i thought I'd just trick this out and see just how far this little baby can go. So if anyone has a schem, could you post it? Then I'd be interested in anyone's opinions about caps, resistors, wire, etc. Vendors are welcome to email me:

raybronk@comcast.net

This way, no rules are broken. I'm gonna get some Teflon Tube sockets. Now whether they make a difference in sound, don't know. So what do you think guys. I don't want to change the part values. There is one mod, about changing the heater voltage from 12 to 12.6V that is supposed to make the GGP sound better. So let the discussion begin.

Ray Bronk

[JoshK]

I'd have to see a schematic before I'd make any suggestions.  I, personally, feel that upgrades are best done yourself, so you can do one at a time and evaluate.  Tedious but more systematic. 

What I've read about undervolting heaters (what having 12V on a 12.6V heater is) is usually positive in favor of the lower voltage (slightly lower, not a lot lower).  It usually means the tube runs a bit quieter and sometimes you get a tiny bit of distortion reduction too, but not always.  It can be a bit tube specific though.  The GGP uses 12au7's? 

Well if I had a schematic I could make some suggestions of things to try, but there would be no gaurantee they would result in better sonics.  However, there is usually enough chatter about good results to make me suggest trying them.

[guest1632]

I'd have to see a schematic before I'd make any suggestions.  I, personally, feel that upgrades are best done yourself, so you can do one at a time and evaluate.  Tedious but more systematic. 

What I've read about undervolting heaters (what having 12V on a 12.6V heater is) is usually positive in favor of the lower voltage (slightly lower, not a lot lower).  It usually means the tube runs a bit quieter and sometimes you get a tiny bit of distortion reduction too, but not always.  It can be a bit tube specific though.  The GGP uses 12au7's? 

Well if I had a schematic I could make some suggestions of things to try, but there would be no gaurantee they would result in better sonics.  However, there is usually enough chatter about good results to make me suggest trying them.

hi ajoshK

On the power supply, the GGP has 6 22uF 350V and on the audio board it has 3 22UF also 350?V caps. Oh yeah, it does have 1 1000uF cap at 25V. Not to sure what that's for. Hopefully someone will post a schematic so everyone can see what can be done. The output coupling caps are 1uF at 600V I think the supply Plus and Minus 200V Yeah, 12AU7 3 tubes and SS rectification.

Ray

[JoshK]

I saw a schematic for the audio portion of the GGP, it was a 6SN7 version of the GGP.  I'll respond more tomorrow...time for bed.

[JoshK]

Ray,

Getting back to you.  The design looks pretty cool from my perspective.  I haven't built and measured so I don't know how it performs compared to some other designs but it looks like it has a number of benefits going for it.  For example, the input is buffered but the topology so it takes stress off the output stage of the source component (no slewing problems), it should be wideband thanks to the grounded grid gain section and the cathode follower top section provides a nice lowish output impedance for driving cables and input sections of amps. 

The coupling cap is the most obvious place to roll your own personal favorite.  It uses a rather big 1uf cap, so maybe just bypass with a .1uf of your favorite uber cap rather than shell out big buck on a 1uf teflon or the like.  I'd also bypass the 'lytic cathode bypass cap (bypass the bypass...ha!).  I'd use a decent metallized poly cap to bypass the 'lytic as technically the cathode bypass is part of the signal circuit.

I might also put a PSU decoupling cap, say 0.1uf right at the tubes from B+ to ground.  Make sure the voltage rating of this cap is nominally higher than the B+ voltage. 

There is a couple of website I saw when searching for the schematic that detail many mods/upgrades/tweaks tried by various owners.  I'd look to those.  My own personal bias aknowledged the first of which I might try is replacing the 12au7 with the 6CG7/6FQ7.  However, you need to change the heaters to be able to provide the 6V's and current differences the 6CG7 requires.  The rest of the circuit is pretty well kept the same from the 12AU7 to the 6CG7.

I am not posting the schematic I found as it seems that Rosenblatz doesn't want the schematic posted publicly.  I didn't find the power supply anyway. 

[Bill Baker]

 I run a lot of my 12 series tube's heaters at 12V rather than 12.6V (6V vs 6.3V) and found I did prefer the lower voltage. I have heard of others running the heaters at 5V (unused tube rectifier taps) with great results but I have not tried this myself.
 Anyone out there try this?

[JoshK]

Bill,

A lot has actually been written about so-called undervolting tubes.  The benefits are said to be quieter operation, sometimes improved distortion levels (very tube specific) and unsupported claims of longer life.  The first is probably the easiest to verify and most likely to expect.  However, I don't think all tubes react equally to undervolting. 

Pete Millett has a nice page on his website about testing undervoltage on some tubes with data and measurements of the results.  www.pmillett.com

I've read some discussion on the topic in diyaudio (tube forum) as well.  I'd search there for some discussion with more empirical evidence. 

There is a difference in running 6.3V heaters on 5V then running 6.3V heaters on 6V. The prior is pushing it and I'd guess quite a number of tubes wouldn't like that or operate correctly. 

[Bill Baker]

HI Josh,
 For those I have actually tested, it was the 12AU7 and 12AX7 that proved to provide a quieter operation with the distortion showing minimal change. As you mentioned, this is tube and circuit dependant and not justifiable in all designs. Lots of variables to consider so it is not simply a no-brainer.

 I would agree that the 5V heater might be streching it a bit which is the reason I have not taken the time to try it out. From what I remember from post of those who have tried it, there were no measurements involved to back up the claims and were basing their findings on simple listening. That was some time ago so there may be more on the subject out there somewhere.

[mgalusha]

The GG responds well to better output coupling caps. Jensen PIO bypassed with a Sonicap Platinum sounded very good to me. I've recently become a fan of the Obbligato caps (diyhifisupply.com in Hong Kong), I suspect they would work well in the GG. It's not real picky about tubes but it did sound very nice with some NOS Mullard's.

The volume control pot does need to be replaced IMO but you're already doing that.

BR is a fan of zener diodes for voltage regulation and I'm pretty sure he uses them for both HV and filament circuits but I'd have to look at the schematic to be sure. If I think about I'll look tonight. The schematic is published in his book "Audio Reality", which he sells on his site.

I also put in some fast/quiet diodes in the PS and added some bypass caps to the PS as well. You can also change the feedback resistors in order to change the gain (about 12dB IIRC) or to give it a "tubey" sound. The GG does not have the "typical" tube sound of warm and perhaps syrupy. The feedback values are also discussed the Audio Reality chapter on the GG.

mike

[guest1632]

The GG responds well to better output coupling caps. Jensen PIO bypassed with a Sonicap Platinum sounded very good to me. I've recently become a fan of the Obbligato caps (diyhifisupply.com in Hong Kong), I suspect they would work well in the GG. It's not real picky about tubes but it did sound very nice with some NOS Mullard's.

The volume control pot does need to be replaced IMO but you're already doing that.

BR is a fan of zener diodes for voltage regulation and I'm pretty sure he uses them for both HV and filament circuits but I'd have to look at the schematic to be sure. If I think about I'll look tonight. The schematic is published in his book "Audio Reality", which he sells on his site.

I also put in some fast/quiet diodes in the PS and added some bypass caps to the PS as well. You can also change the feedback resistors in order to change the gain (about 12dB IIRC) or to give it a "tubey" sound. The GG does not have the "typical" tube sound of warm and perhaps syrupy. The feedback values are also discussed the Audio Reality chapter on the GG.

mike
 

Hi Mike,

Well, I have thought about those feedback resistors. Not sure I'm even gonna touch them. As far as tubes go, I've been told that the Cleartops RCA are good and reasonably cheap.

So what caps did you use for the bypassing and what diodes did you use? Was thinking about some hexfred There are 6 22uF caps on the supply board, and 3 22uF on the audio board. Not sure what those three do, but don't want to do anything that's gonna change freq response.

Hey Bill, what VCap would you recommend for the output coupling cap. The present ones are 1.0uF either at 400V or 600V like i said I want to just go all out and see where this can go. 200 bucks for a cap is a lot of money, but if that's the best then I'll get it. What about the Teflon tube sockets? Don't want to get them if they won't do anything good other than grip the tube pins.

There was one guy who said changing the heaters to 12.6 actually opened up the sound more. Don't know about that one. That would take some mod of the regulation circuit on the GGP. If I go that route, I might as well get the CCS boards from Bottlehead.

People seem to think the 12BH7 is a better tube than the 12AU7. It does I'm told require a modification of the heater pin replacement, but that's also a slight possibility. At this point, I'm just interested in upgrading the present parts to better ones. Maybe, get some 5751, or some 5814 tubes. Any comments? I don't know if those 3 caps are coupling caps between stages. To bad, those just can't be removed.

Ray

[JoshK]

I think the 5751's are 12ax7 equivalents, iirc. That would be way too much gain for a preamp. 

[guest1632]

I think the 5751's are 12ax7 equivalents, iirc. That would be way too much gain for a preamp. 

They are from what I understand less gain than the 12AX7. Now if I wanted more gain, then it's a matter of changing out a few resistor bvalues and I'd get 30 DB as opposed to the regular 12. Could also remove the feedback resistors entirely, which would bring the gain up to 19DB as opped to 12.DB. I think that's one place I'm not gonna touch.

Ray

[pbrstreetgang]

5751s are 12ax7s with 30% less gain

[mgalusha]

Well, I have thought about those feedback resistors. Not sure I'm even gonna touch them. As far as tubes go, I've been told that the Cleartops RCA are good and reasonably cheap.

So what caps did you use for the bypassing and what diodes did you use? Was thinking about some hexfred There are 6 22uF caps on the supply board, and 3 22uF on the audio board. Not sure what those three do, but don't want to do anything that's gonna change freq response.

There was one guy who said changing the heaters to 12.6 actually opened up the sound more. Don't know about that one. That would take some mod of the regulation circuit on the GGP. If I go that route, I might as well get the CCS boards from Bottlehead.

People seem to think the 12BH7 is a better tube than the 12AU7. It does I'm told require a modification of the heater pin replacement, but that's also a slight possibility. At this point, I'm just interested in upgrading the present parts to better ones. Maybe, get some 5751, or some 5814 tubes. Any comments? I don't know if those 3 caps are coupling caps between stages. To bad, those just can't be removed.

Ray,

It's been several years but I'm pretty certain I used HexFred's in place of the 1N4007 diodes. As for the 3 22uF/350V caps on the main board, they are part of the PS, just located closer to the tubes. I wouldn't remove them. I bypassed them with some small caps but I don't recall what. I believe I used some small Xicon polypropylene caps. As Josh mentioned, be sure they are rated for a high enough voltage. The rails are +/- 200V, so I'd want 400V caps to be comfortable. Mouser has Wima .1uF polypropylene for $.99 each. At the least bypass C7 and C11, these are the caps that filter the zener noise and getting that lower is always a good thing. The zener's are the high voltage regulator.

There are no coupling caps between stages, the only cap in the signal path is the output cap, the 1uF/400V cap on the output.

As mentioned earlier I said I'd look at the schematic and he just uses a regular 12V regulator. If you really wanted to go with 12.6 volts you would need to replace it with something like a LM317 and the associated circuitry. It wouldn't be hard to build it on some perfboard assuming there is room to fit it in. I never bothered with that but it shouldn't be difficult.

I have some RCA clear tops and IMO the NOS Mullard's were a better choice in the GG but that was just me, others will feel differently.

mike