** Peavey Mace VT Screen Grid Value Options **

[mrr3000gt]

I have one of these on the bench and have ordered the 100 Ohm SGR to Peavey's spec.


What harm does using 1.5K/5 Watt have for this amp? Its the six 6L6 power tube model and runs at a nominal 500 volts (490 to screen grids).


Will the tone and sound suck with 1.5K? This amp uses 47K grid stoppers.


I ask because to me 100Ohms seems a really small value. I also have a bunch of 1.5Ks..

[sluckey]

No harm. I doubt anyone has ever done this to that amp. You'll have to be the guinea pig. Let us know what you think.

[shooter]

have ordered the 100 Ohm

What harm does using 1.5K/5 Watt have

which one are you gonna use 

going from 1.5K to 100 ohms at 500vdc will probably shorten the tubes life, how much, measure the increased current n do the math

Will the tone and sound suck

I'm only qualified for linear math 

[mrr3000gt]

I have never heard one - I only heard a Duece I had and it was in rough shape. So if its a bad change I will never know it. It had been repaired before and I kind of thought it was the 100 Ohm 'fuses' they were using for SGRs.


I just don't want to dull the sound or reduce the response to garbage. I did hear that these amps can use EL34s but able to use EL34s does not mean you should. The amp seems designed around 6L6s.

[mrr3000gt]

The amp is stock with 100 Ohm SGRs and 47K Grid stoppers.


I ordered the stock 100Ohms, but have plenty of 1.5Ks on hand and was not sure if this would make it into an amp nobody wants for the extra security the 1.5Ks will give.

have ordered the 100 Ohm

What harm does using 1.5K/5 Watt have

which one are you gonna use 

going from 1.5K to 100 ohms at 500vdc will probably shorten the tubes life, how much, measure the increased current n do the math

Will the tone and sound suck

I'm only qualified for linear math 

[sluckey]

If you are a business it would probably be wise to stick to the stock Peavey plan.

[mrr3000gt]

Totally agree - This one is a special case and I might snag it for myself.


I do not experiment on any contracted work or on work for music shops as a matter of practice.

[PRR]

On 6L6, 1.5k should not cause any "suck", except slightly reduced damage in gross overload.

EL34 would be more sensitive to large screen resistor. On a EL34 jazz amp the slight suck-down of 3k was audible but pleasant.

[glass54]

Dear mrr3000gt,
I haven’t worked on one of these for the best part of 20 years (not many Deuces/Maces in Australia). Just make sure that the bias supply is sound and careful not to run the tubes too hot.
My (long winded) thoughts regarding Screen Resistor values. I have repaired numerous Marshall 9200 Power Amps, usually 4 off EL34 but sometimes 4 off 5881. The EL34 versions have 1k 5W Screen resistors and the 5881 versions have 470 5W resistors. Occasionally I get asked to convert the EL34 to 5881 tubes which includes a set of resistors and rebias (5881 typically around -48V on the 9200). The 5881 is approx equivalent to a 6L6GC and is rated at 23Watts Plate dissipation vs 30Watts for 6L6GT. I did a little more research and noted that Older amps had no Screen resistor (Fender Twin models 5D8 and 5E8 using 6L6G tubes), 100R in Twin model 5F8 (using 5881 tubes) and 470R in Twin model 6G8 (using 6L6GC tubes) and again Twin Rev AA270 (using 6L6GC tubes). Similar trend in Fender Bassman models. There was a similar trend in Marshall as well, model 1987T had no screen resistor but the model 2204 had 1k screen resistors! But the JTM45 (for types 1961, 1962, etc) had a 1k (post inductor) feeding two 470R to each screen on KT66 tubes!!
So can we assume that historically Screen resistors were introduced to tame the tube destruction phenomena by reducing/limiting current in G2? Obviously there is a compromise between increased life of tube vs increased distortion by excess value of screen resistor.
I believe Tubeswell and others has some good info here: https://el34world.com/Forum/index.php?topic=21489.0
And another point to consider: “Since screen current affects the tube's operating point along the transfer curve and changes the shape and slope of the tube's transfer curve it has more effect on power tube overdrive tone than grid current”
….and finally, as Sluckey said, give the 1k5 a go and please report back to us to with your results and thoughts on the change.
I also agree, not a good idea to experiment with customers (rarer) amps
Kind regards
Mirek

Update: Just noticed that some manufacturers eg JJ (T-6L6GC-JJ) and "updated Tung Sol" (T-6L6GC-STR-T) have plate dissipation of 30Watts for 6L6GC   

[mrr3000gt]

Thanks guys!


I do know that the response and handing of a 6L6 screen is different than the RL34. I can do the math and in the end I am faced with the question: “does the difference matter” and “is the sound different and if it is - is it a bad sound?”


I was not sure if 1.5k was into the zone where it mattered for a 5881/6L6 circuit.


One question: is 100 ohms functionally help and do anything other than a minuscule voltage drop? This amp is known for snappy and responsive clean tone. The Knopfler brothers both used the Mace back in the VT version days. I would hate to drop in what my engineering hat tells me to do to make the circuit more robust to in the end have a pedestrian sounding amp...

[glass54]

Its a gutsy amp with good headroom (see attached specs)
I suspect that all you would achieve with 1k5 resistors is a little compression near max power.
But you have probably decided to maintain a stock (historic) amp as it is a customer's item and not an experimental amp.
We were looking forward to the results with 1k5 resistors 
Kind regards
Mirek

[mrr3000gt]

For what it is worth: This is the schemo. I have to admit it is a little strange working on an amp with 6 power tubes as the last time was with an early SVT head.


Nothing much to see in the design - the first Mace did have adjustable bias whereas the VT and VTX models shortly after did not. Power section is as simple as you can get and the PI is nothing more than a pair of transistors. The voltage is not crazy high like on a SVT. The screen voltage is 495 with the nominal voltage at 500.


The build is rather beefy and the quality not unlike Marshall JMP and early JCM800s I have worked on. One of the issues I am fixing is the crappy repair did to the power board. Looks like a lazy repairman wearing oven mittens worked on it last. At least 30% of the repair work I ever done was fixing crappy workmanship from the slop shops.


I am not much for mods - 20 years ago I started discouraging mods to old and vintage gear. I figured if an old Fender Twin or similar survived the many decades of owners wanting it to be something it is not rather than just buying what they wanted that I would help preserve the original circuit. Guys like Andy Fuchs are the exception of course :-). Like bad workmanship in crappy repairs from slop shops, another 10% of all the work I ever do is mod reversal.


The clean tone and headroom of the Mace is supposed to be crazy good - enough many musicians (that were not southern rock icons getting free endorsement equipment from Peavey) used this amp. The Knopfler brothers supposedly dumped their Fender twins for the VT series Mace when they started going on the road for big shows. I have a '65 RI they will probably bury me with so I want to see what the deal is :-)

[Platefire]

I bought one of those new in 1979 long before ever getting into tube amp mods/building/repair. I think the main reason I got it was I was a big fan of Lynyrd Skynyrd back then as I think Gary Rossington uses one up to this day. The board smoked on mine while it was still under warranty and I sent the chassis back to the peavey factory and they sent it back like new. Never had a problem with it after that until I sold it in the mid 80's. I never felt at home with it and always yearned for my old Silverface Bandmaster I got rid of.I think it could probably run even or out run a twin on a weight race. Ya'll forgive me, the old man has to reminisce every now and then. Platefire 

[mrr3000gt]

Nice to hear from you Plate! This is like the Blue Voodoo BV60 I had from the other forum - total sleeper amp.


My Duece I had look to have had atleast 1-2 fires on the board. The new Mace looks like it was only repaired from cheap tube burnout (power section, sloppy repair,..). The insides actually look good and I am thinking its a border model between the end of the Mace 320 and the start of the VT/VTX series. This one has BW badging and Black Widow speakers, but unlike the early VX's has the classic Peavey colored knobs (and not the 70s silver top of the 320 run).


If I had to guess on a date I think 1979/1980 sounds right. Crate was similar to Peavey inthat some model changes overlap within the run (Mesa did the same thing with the Son of Boogie model amp).


I am going to make it stock and see how it works. If it seems good and not a sink hole of time, money, and effort I will snag it and restore it (more).


As far as the Lynyrd Skynyrd connection: The VT and VTX are as close to what Gary used as the JCM800 and JCM2000 are to the JMP series amps. Close amp siblings: Gary and the boys used the Mace 320T and this was a little different than the VT Classic or VTX models of the Mace. The vibe is nearly the same but a 320 is what you need to have the same tone (and his hands, guitars, and playing style of course).

I bought one of those new in 1979 long before ever getting into tube amp mods/building/repair. I think the main reason I got it was I was a big fan of Lynyrd Skynyrd back then as I think Gary Rossington uses one up to this day. The board smoked on mine while it was still under warranty and I sent the chassis back to the peavey factory and they sent it back like new. Never had a problem with it after that until I sold it in the mid 80's. I never felt at home with it and always yearned for my old Silverface Bandmaster I got rid of.I think it could probably run even or out run a twin on a weight race. Ya'll forgive me, the old man has to reminisce every now and then. Platefire 

[PRR]

> Just noticed ...dissipation of 30Watts for 6L6GC

6L6GC replaces 6L6 but is not an original-flavor 6L6.

807, cut-down is 6L6. Released as 21W and quickly walked-back to 19W. Improved materials made 21W-23W ratings on several of the G types.

Designers were confused, and greedy, and worked original 6L6 too hard. Tung-Sol promoted the 5881 for sockets which were eating 6L6(g) too fast. Ratings were not a heap higher but a much beefier tube. (Whatever sells as "5881" today is not made the way old Tung-Sol made tubes, and is probably a Russian copy of 6L6/6L6GC-types with "correct" 5881 bottle and marks.)

TV designers liked the 807, but not its price, and soon over-ran its ratings. By now vastly improved materials and tooling allowed a clean-sheet re-design. This gave a top-cap (BG?) for 807 apps and also the audio 6L6GC, which generally replaces all prior "6L6". Carries a 30W rating, which may be conservative in cold-bias speech/music, but in hot-bias amps I like to stay well below 30W because I have seen grid emission. (May depend who made them and when.)

[mrr3000gt]

One of my most favorite 6L6 tube variants is the 6Π3C-E.

You might know it by its other names: Sovtek 6L6WXT or 5881WXT. This tube has always been a go-to 6L6 for me not because of the tone or sound. The tube is bullet-proof and I have not seen many burned up. Somewhere I read the dissipation is rumored to be along the order of 100 watts where a standard 6L6GC is 30. I have never tested that (or totally believed it) but these tubes work great and sound good enough for a working musicians amp.


This guy gives a great write-up on the tube:


http://www.jayskyler.com/guitar-gear-guide/sovtek-5881-wxt-6l6-wgc-tube-guide.html


The Mace I am working on came to me with a set of 2006 Sovtek 5881WXTs. I checked them on my trusty Sencore mightmite and found one had a leaky grid (bad) and the others tested good and strong with great life. Not bad for a used 2006 set of tubes. I also found that two of the screen grid resistors I pulled out measured not at 100 ohms but ZERO ohms. So something catastrophic happened to the amp.


In my experience what I see with the tested values of the tubes tells me they are still good. I had had sets of Sovtek WXT pulls from various repairs over the years that I used for many years. I am thinking the leaky grid tube probably failed and took the two SGRs with it. And the crappy repair looked to be many decades old.


And it looks like the previous owner retailiated (on the amp) as the power chord was pulled out and the fuse assembly was totally destroyed. I replaced and rebuilt everything so it looks good. Whats funny is this was a stage amp at a church for the life of the amp (since it was new). I want to get it together and play some nice heathen southern rock with it to see if that makes it happy hehe.


The power and bias are strong and healthy on the amp and I am thinking with the new 100R/5W SGRs these tubes will let me test the work until I get a descent set of tubes.


I am keeping it: so SGR experiments later might be in order.

[mrr3000gt]

So the resistors came in and got everything fixed - sounds great! I read the reference thread suggested by Glass about screen/grid resistors and have to agree on the utility of the standard values for both for each of the tube types. Between battling the natural Miller capacitance and being a rudimentary safety net for the tube there seems to be very little latitude required as most of this has been figured out through the decades.


The Mace circuit has tons of headroom and it is my opinion that the noticeable effect of a 1K-1.5K would not start to happen until the volume was so loud it would not matter to the listener. The amp is LOUD. All voltages and tube states were within nominal. I ran the amp outside of the case instrumented with various meters I have and I observed changes in tube current and voltage (and grid voltage activity). Its a good thing the two Black Widow 1201 speakers are rated at 700 Watts RMS each (and no less are run in parallel at 4 ohms for the amps maximal output - 1400 Watts RMS???).


The amp seems to operate like a Mesa S.O.B. I had: bone chilling cold bias at idle (low volume, no input) and under attack of playing loud and hard it jumps in current draw to what seemed to be about 17-19 watts per tube. I had stuff rattling off my workbench and it was getting loud. I guesstimated even at maximum power the peaks would not be anything the amp could not handle. Even now my ears are whistling but the definition was good even loud. And very Skynyrdesque - the neighbors down the street got to hear Simple Man for a bit while I made sure the work I did was good.


A most strange amp with its combination of design, power, and specs. Certainly built to be robust for sure!

[glass54]

Hi mrr3000gt,
(Just to be a pain) the ratings on the Black Widow 1201-8appears to be "Power capacity: 1400 W Peak, 700 W Program, 350 W Continuous". So you have a nominal 700W RMS 
See the link for more https://www.amplifiedparts.com/products/speaker-peavey-12-black-widow-1201-8-bw-700w-8
Glad the amp worked out well and we all learnt a little about 5881/6L6GC/6L6GT and other variations.
Kind regards
Mirek

[mrr3000gt]

Thanks for the clarification :-)


With the two of them in parallel makes for lots-o-watts. I learned that with low tube wattage you can afford to use small values for grid/screen resistors even when the power output is a major part of the equation. I think if your objective is to have (a) a reliable and maintainable, (b) more amp output than normal then tone must suffer as getting all three seems generally difficult.


Low values of SGR and the normal neglect I see on amps in general tells me in particular they expected the owner to be close enough to a shop for maintenance - like another piece of pro gear you expect to use. I have only seen these after years of real use (gigging) and many decades old. We won't see that years from now with the crop of junk being sold as musical gear these days: low cost and programmed obselesence strikes again.

Hi mrr3000gt,
(Just to be a pain) the ratings on the Black Widow 1201-8appears to be "Power capacity: 1400 W Peak, 700 W Program, 350 W Continuous". So you have a nominal 700W RMS 
See the link for more https://www.amplifiedparts.com/products/speaker-peavey-12-black-widow-1201-8-bw-700w-8
Glad the amp worked out well and we all learnt a little about 5881/6L6GC/6L6GT and other variations.
Kind regards
Mirek

[mrr3000gt]

Its been about 6 months since this threat has been alive so I'll revive it.


I ended up snagging the Mace for my own. its as restored as I can reasonably get it. I went with the stock SGR (as earlier stated). Its out of 19th century science fiction with its 6 power tubes in a massive row. I did not have 6 matched tubes so I cobbled together a 4+2 set of ancient pulls I had. Sounded awesome believe it or not!


I did get a nice set of 6 matched 6L6WXT+'s for it and it sounds even better as one would imagine. It is loud when you need it loud, but it actually can sound downright good at lower volumes with infinite head-room.


It is every bit a hernia machine, but so is my 1965 RI Twin!

[Custom_Amp_30]

Here is my Peavey Mace, Head unit only. Trying to get some info from you about the exact speaker cabinet needed for this amplifier. I see the 160 Watts listed as a 2 Ohm impedance setup, so how was the cabinet wiring actually wired? I'm still looking for the cabinet to make this a complete unit. The specification page is from the front cover of the service manual.
Any advice, leads or input would be welcome. Thanks

[PRR]

> I see the 160 Watts listed as a 2 Ohm

Where do you see that?

The plan for all three versions(?) shows Main being a 8 Ohm connection.
https://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-Mace-Deuce-VT-Schematic.pdf

[glass54]

Hi PRR
The owners manual is the location. I'm attaching Page 1 as the full manual (pdf) is approx 1.3Mb.
If you need the full manual, please let me know 
Kind Regards
Mirek

[AmberB]

The specs also list 160 watts for a four ohm load.  A 4-12 cabinet with 4 ohm speakers wired in series-parallel will meet that need.  For a 2 ohm load, all four of the 8 ohm speakers would be wired in parallel.  You could also use four 16 ohm speakers wired in parallel for a 4 ohm load.