Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on June 17, 2015, 03:51:41 pm
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I converted this 1976 Peavey Pacer to a One Channel AB763 Deluxe Reverb with a 6G16 Bias Tremolo in 2008. The amp is what I would call unbearably bright that when you tone it down with eq I end up cutting the bright switch, boosting bass and cutting treble to get in the tonal area I like. When you eq to that extreme the amps tends to get a little boomy. The mid eq control is a combination mid/raw. Up to about 4.5 mid peaks out and the tone stack is then progressively dialed out to fully clockwise(no tone stack). Also the amp tends to be a little sterile in touch response. Playing it fully raw helps the sterilness but is usually too bright like that also. I have tried a few other speakers but the brightness/sterileness seems to follow. The plate voltage was a little hot last time I checked at about 415 VDC on two JJ 6V6S. The bias on those tubes reads 26mV and 30mV which should be hot for 6V6's but are relatively cool to the touch. I've tried to love this amp as is but my love is running thin;>/
That is the main issue with the amp. I have a couple of minor issues that I think is easily tweaked which is on the bias trem the speed control works fine but intensity has to be at 100% to get close to what I like and still comes up a little short. The intensity needs to go more intense.
Also the amp has a little more operational hum than I like that needs to be lowered.
I am attaching the as-built schematic, the original Fender AB763 DR schematic for reference and a gut shot for an idea of what the layout is. I might have some mistakes I never located??? I would appreciate your input, suggestions and pointing out any errors you see. I want to get this amp usable for my purposes and would appreciate your help. Platefire
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Different speaker?
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Yeah, I have tried different speakers but ones I had on hand and didn't order anything new. I originally had a Carvin British Series which was made by Eminence, and Texas Heat by Eminence and right now I have a Weber Ferrowmax/VST 12SIGF-S. I recently took out the vintage RCA Blackplate in V1 and installed a new EH 12AX7 and it seemed to help some. It just seems strange to me having to go to such an extreme on the eq to dial in some fat tone. Maybe that's normal for this amp? Platefire
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I did some more experimenting to try to eliminate the basics before I pulled the chassis. I checked all the tubes:
1-Tried a vintage set of Sylvania 6V6's and it had no effect on the tone.
2-Traded out all of the preamp tubes one at a time to see if that made a difference. It really didn't.
3-Tried a Eminence Ragin Cajun speaker, no improvement.
4-I even tried running the bias a little cooler, no improvement.
So I'm beginning to believe the fault is in the circuit.
I have found a couple of mistakes on my schematic. Phase inverter tube should be a V5(12AT7) instead of V4 and the V4 should be the tremolo tube presently labeled V5.
I know I did this when I was in completion stage back in 2008, but I think I will do it again and go over all components for correct values when I pull the chassis. Platefire
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Have you tried the amp without the nfb-loop?
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Hi Platefire, It looks like you have too much negative feedback. Leo used the 820 ohm /100 ohm circuit for the 2 or 4 ohm speaker load amps. He used 820 ohm /47 ohm circuit for the 8 ohm speaker load on the Deluxe Reverb.
--craig
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I haven't tried it without the NFB. I recall when I had a thread regarding this amp back when I was building it there was discussion about the 47 ohm to ground on the end of the NFB loop---as I recall some AB763 on other models may have had a 100 ohms but the DR had 47 That will be something I'll look at when I pull the chassis and make sure the right value resistors are in there and maybe disconnect it just to see what that does. Seem like a temporary jumper over the resistors might do that?
I keep wondering about that added mid/raw control, maybe jumping over that 100K pot with a 6.8K resistor to ground to see what that does.
Another thing I'm wondering about is my schematic shows 1500 resistor on V1b cathode and the original AB763 DR schematic is a 820--wondering why I did that?
One thing I was noticing last night in experimenting was when I increased the reverb the hum also increased! I tried a different 12AT7 driver and 12AX7 recovery but it didn't change the hum.
Another thing I should say about this just info purposes, when I was checking the preamp tubes last night I found I had a 12AX7 in the PI position. I tried a 12AT7 in that position and it even sounded worst, so I put the 12AX7 back in there---so that seemed a little strange. I think that's what I originally did was tried the 12AT7 and didn't like it and went with the 12AX---this may be an indicator of something amiss.
No doubt about it the amp is unnaturally brittle. Got to get to the bottom of it. I like nice round tone. Platefire
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as I recall some AB763 on other models may have had a 100 ohms but the DR had 47
That's correct. Any model that had a 8Ω speaker used a 47Ω. Models that had a 2Ω or 4Ω speaker used a 100Ω.
Seem like a temporary jumper over the resistors might do that?
Yes, or just disconnect the NFB wire at the speaker jack, whichever is easier.
my schematic shows 1500 resistor on V1b cathode and the original AB763 DR schematic is a 820--wondering why I did that?
Because your V1B does not share a cathode resistor with another tube like the original. 1500Ω is correct for your circuit.
No doubt about it the amp is unnaturally brittle.
The AB763 circuit is capable of sounding very bright because of the mid scooped tone stack. But you should also be able to get mellow tones just by twisting the knobs and turning the bright switch off. If you have any Celestion speakers try them.
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Howdy Sluckey
It's starting to come back to me now. I found my old project folder on this and it was Doug who paved the way on this one on a old fender amp he re-worked including adding the 6G16 bias Tremolo to the amp. I'm thinking there has to be some kind of bad connection, wrong component value or something is out of whack---I just have to fine it. I'm more familiar with fender circuits than anything else and this one is just not behaving correctly. I do have an 8 Ohm speaker--so I need to be sure that resistor to ground on the NFB is 47---not 100 like on the schematic and try it without NFB--may could make it switchable if it has a good effect without NFB?
As I recall it seems we made efforts to make the 1 channel operate as if the other channel still existed. Closer checking may show what? Platefire
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Plate,
After looking closely at your posted pictures I only have one question that may help.
What is the actual value of your slope resistor on your tone stack?
Kinda looks like brown-black-brown which would be 100 ohm instead of 100K :dontknow:
The pic resolution isn't great and it's throwing me off.
Good luck and hope you find the problem sooner than later.
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Thanks Silvergun
I checked that and it's Brown/Black/yellow and also checked it with an Ohm meter and it read 105K.
I think I might have found a serious problem on the wiring of the Mid/Raw control. Please check my as built schematic if it shows the correct wiring for this pot. If I'm reading it correctly the wire from the ground term on the bass pot goes to the input of the Mid/Raw pot, the input and output terms also are connected plus the ground term is wired to ground.
On my amp it's not so----The wire from the bass pot ground term goes to the output of the Mid/Raw pot, the input term is empty! and the ground term is wired to ground. So it the schematic right on my amp wiring??? Platefire
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Is the pot order on the chassis
Volume
Treble
Mid
Bass
If so, look at a fender layout with a i mid pot as it should be the same.
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I have had a Deluxe build feel sterile from too much voltage on V1 and NFB can always do this. When you determine you have no wiring issues and it is still sterile. Disconnect NFB as suggested and if you have a Variac try lowering the voltage to the amp. Of course check v1 first. Mine was at about 190vdc and dropping it with a 5u4Gb gave it some sag and give. Touch!
I now has a 180K Plate resistor on v1 as well.
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Like this Plate;
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I think I might have found a serious problem on the wiring of the Mid/Raw control.
You're fine. All three of the schematics in this pic are the same even though they are wired slightly different. Your mid/raw pot is being used as a two terminal variable resistor.
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Thanks guys for all the help and input. I've been working on it about the last 3 hours. My Middle/raw pot was wired like sluckey "A" arrangement even though the my schematic shows type "B". I got an alligator clip jumper and wired it up like sluckeys "B" arrangement and to my ears it sounds a whole lot better. I know sluckey said it is no difference, just a variable resistor to ground but when I including that input term like "B" arrangement, it made a world of difference to me. If it's just my imagination--Great! Good imagination :icon_biggrin: Tomorrow when I wake up, it may sound terrible :dontknow:
I checked all the voltages and included all the new voltage readings on the revised schematic attached. Some voltages on the preamp tubes is as much as 30V to 40V higher than Fender schematic. See V3 at about 210 and should be 170/180. Also on V4 PI more that 50V higher than Fenders. It appears I need to go higher on the power rail resistor to more than 10K.
To me the 12AX7 in the PI V5 sounds better than a 12AT7. Please take a look at the voltages and what do you think? Platefire
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I like lower voltages too. I used a 5R4GYB and my voltages dropped about 10% compared to a 5AR4. It only uses 2 amps of heater current so most power transformers can handle it.
Any update on the value of the bottom feedback resistor? I'd kinda like to see that is right before you start modding to fix the sound.
Craig
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Yeah, I need to change the NFB resistor to a 47. I'm also wanting as Ed suggested to hear the amp without NFB. I'm just wondering if I install a SPST switch to disconnect NFB does the 820 and 47 need to be switched off or just the 820? The bottom leg and 47 to ground is connected to that resistor/cap network on PI, so that makes me think only the 820 would need to be turned off---any insight? So I think just disconnecting the NFB from speaker jack would do it!
Also I need to look through my tube inventory to see what rectifiers I have. Currently a GZ34 in there.
On the tremolo, I changed the the 470K resistor to 220K and the .1 cap to ground to .047 adjacent to the intensity pot to increase intensity back in 2008 right after I finished but--I have to turn the intensity to 100% now to get what I want, so to get a little more play in my pot travel and a little more adjustment, I need to change the 220K resistor to 100k as instructed on Doug's tweaking of the 6G16.
Also to decrease operating hum, think I need to go from regular hook up wire to shielded on the input jack to first stage preamp and then from V1a first stage preamp to also from V2b second stage preamp where hookup wire goes under board to volume control. If I still have some hum after that, I will know to look elsewhere. Thanks, Platefire
BTW-checking my tube inventory, I do have a couple of 5U4GB's but not anything else that would work. How bout 5Y3?
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You can put the switch in series between the 820Ω and the speaker jack to open the circuit, or you can put it in parallel to the 47Ω to short it out. Either way works fine.
I have to turn the intensity to 100% now to get what I want
I noticed your output tubes are biased on the cool side. If you bias them hotter you will get more tremolo effect. There's always a tradeoff between bias and trem strength in this type circuit.
5Y3 may be too light weight for a Deluxe Reverb. But I'd try it. If you have a NOS 5Y3 it will definitely drop the voltage. I would not put that 5U4 in it unless you know the PT can supply 5volts at 3 amps. 5AR4/GZ34 is what Leo used. I'm not concerned about the voltage on the little tubes. Anyway, if you crank the bias current up to about 25mA, the extra load will likely lower all plate voltages in the amp. Try it and see. My DR is running with 423v on the plates and 24mA cathode current. You can see my voltages here...
http://home.comcast.net/~seluckey/amps/tdr/tdr.pdf (http://home.comcast.net/~seluckey/amps/tdr/tdr.pdf)
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sluckey
I did the bias change as you suggested. Even though my JJ's are supposed to be a matched set, their off, so I had to find a happy medium---a little below and a little above 25mV. So it's presently set at 23.9/27.6 and -36.5 Bias voltage.
I re-checked all the voltages to see what effect it had. All seems to be in the ballpark as yours with the exception of the PI, which is a 12AX7 in mind instead of a 12AT7--I don't know if that's why they so much higher? Anyway here is a side by side comparison:
sluckey Platefire
V1-206/203--------V1-206/205
V2-424/424--------V2-414/412
v3-206/209--------V3-203/215
V4-196/194--------V4-242/220
V5-423/425--------V5-420/419
V6-423/425--------V6-420/417
V7-330/330AC-----V7-329/329AC
Har! Got a question about your schematic;>) You said in an earlier post that the NFB resistor to ground should be 47 with an 8Ohm speaker and 100 for 2/4 Ohm load. So why do you have a 100 Ohm resistor with your 8 Ohm speaker :w2: Thanks, Platefirre
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So why do you have a 100 Ohm resistor with your 8 Ohm speaker
It's a mistake.
Did your tremolo get stronger?
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Regarding the "is the tremolo stronger" question? a long story!
Before I started tweaking my amp this time, I had the bias really hot closer to 30mV. I got to reading voltages and checking with the Weber Bias Calculator and ended up adjusting it down closer to 20mV. So my statement that the intensity needed to be slightly more was was in reference to the old 30mV setting before I started all this tweaking. So no, it's not better but about the same and really its almost adequate but it's just something about turning it to 100% without anymore play---bugs me.
OK---on the 47 Ohm. Just checking! Have you ever tired yours without the NFB? Thanks, Platefire
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Have you ever tired yours without the NFB?
Yes, but I didn't like it. The amp sounded more open and raw. Lost a lot of that blackface Fender vibe.
If you put a 22µF bypass cap on the cathode (pin 3) of your trem tube the strength will increase considerably.
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Hay sluckey, thanks for the tip on the 22uf/25V on the trem cathode resistor. I just temporarily tagged one onto the resistor legs with a good mechanical connection for testing and yeah, I think that helped to where I can reach my fav sound at about 3:00 with a little play.
I went ahead and disconnected the NFB and tested. I like the added option of early dirt and Man! turn the raw fully clockwise with the NFB off and it's a loose cannon:>)----the good thing is with a switch you can easily get back to blackface territory. Same with the raw control--turn it counter-clockwise to about 4.5 and your there. So I don't mind drifting out of blackface territory as long as I can come back home.
I did notice when I disconnected the NFB, the operating hum increased with more gain. Not all that bad but it's on the edge of aggrivating. I checked all my ground points--good, appears to have good separation between power supply and grid wires. Not sure what generating the hum---again first step shielded on grid wires up to vol pot. Thanks, Platefire
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Plate,
IF you want a smoother overdrive that is not brittle or harsh in any way, consider giving this a try. I think you would find it smoother and a clearer (less muddy/gritty) tone. I find both the PAB (preamp boost) and the mid-boost to be quite useful.
This is essentially the tone stack I use on my Tweed BluezMeister. I feel like it has a wonderful clean tone that is significantly clearer and warmer then the Princeton Reverb I owned and most AB763 amps I've played.
I find the 25uf cathode caps to consistently be too muddy for my tonal preferences except in rare exceptions. .
with respect, Tubenit
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Tubenit,
What are those two symbols within the highlighted section that look like traffic lights? (The only things that aren't shaded yellow).
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SPDT mini-toggle switches. I can either switch mine by mini-toggle and/or relay foot switching.
With respect, Tubenit
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Hi Tubenit
Thanks for the plan. I went over it as a possibility and one thing I couldn't work out is real estate to locate the two SPDT switches---the back panel is the only space left. I was also going to put a NFB switch back there---so would one of these switches in your plan eliminate the need to switch NFB off for more dirt or to be used in conjunction with the NFB switch?
I'm going to be out of pocket for a couple of weeks so I will have to resume this when I get back. Something to ponder :think1: Thanks, Platefire
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Plate,
I have done some negative feedback switches in the past on different amps. It's a useful mod and I do like the tone of the NFB disconnected. However, it does induce more noise into the amp at idle and has more grit then smoothness to it. So, I have not done one on my last several amps AND I removed them on two other amps. On one of those amps where I removed it, I simply inserted a plastic plug to fill the hole. So, I wasn't removing the NFB to make room for anything else. I removed it because I wasn't using it.
I do like the PAB and midboost on my amps. I've put something similar on 5 different amps and found them worthwhile. Between those two, I use the midboost more. The PAB feature helps a lot with overdrive but it induces noise at idle like the NFB switch does. So, .............. IF I only had room for one mod, I'd do the midboost. The midboost is especially useful going from strat single coil type tone to a fatter humbucker type tone.
Remember, all of this is personal taste & what I like might seem less than useful or desirable to another person.
What I do like a lot is how the mid pot and bass pot are separate and each how their own ground. To my ears, this gives more tonal options as I dial those then when the bass is connected into the mid pot like on Fender amps or Marshall. AND this is a very easy rewiring to try out.
A fairly non-intrusive and easily reversible wiring approach may be to replace the cathode caps with values I suggested and simply wire your mid pot and bass pot apart and use a 10k resistor to ground on the bass pot. IF you like that, then add the mid boost feature.
Note the mid and bass capacitor values in the tone stack I use. It may be helpful to try those, but what you have already may suit you just fine?
Anyhow, these are just ideas to consider. I want your amp to sound like what you want, so I am just tossing out ideas in response to the thread comments. Not saying you "should" try this at all. :icon_biggrin:
Best regards and respect, Tubenit
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Well I did determine something last night. I agree with you about not having a NFB switch. A couple of days ago I hooked a SPST switch up in the back of my chassis to the NFB without drilling a hole and have been A/B'ing it with and without NFB. Tested it pretty thoroughly with a lot of jamming. I too believe it's something that would go by the wayside into non use.
I am kind of leaning to preserving the original circuit since it's sounding so much better to my ears when I rewired my raw switch like my own schematic. Don't know why it wasn't wired that way to start with? I think I really need to get a better speaker for this amp that would lean in a darker direction instead of brighter. sluckey suggested Celestion plus I still have some corrections to the circuit I haven't performed yet I probably need to do first before making any further determinations. Since I don't have any old vintage fender amps, my original idea was to get as close as possible to that sound with this Peavey Pacer conversion. I have seriously considered removing the Mid/Raw control and putting the tone stack back to original spec--I can't help but wonder if that slight deviation might be hindering it. My only historical reference tp DR sound is I did have a Silverface DR in the 70's that I really liked. Platefire
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Did someone say Celestion? Greenbacks do sound nice in a Deluxe and I have used a single 12 Blue extension. I really love the compression of AlNiCo speakers in Fender amps.
The Blackface Deluxe is arguably the most desirable club amp there is. It is very versatile in its stock form. I don't care for NFB switches, but I do like them on pots. This way if I am in a small place I can back off and get a little breakup sooner, but if I need more volume and headroom I can tighten it up. IMO NFB is the most important factor in the feel of an amp.
I do like a mid pot and a dwell. Everything else is just icing on the cake. I also like a 2, 10 with a BF Deluxe. It is really a great amp and I am sure you will get it where you want it. I know when I have it right is when I plug in and start playing and no matter what I play it sounds good to me.
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Here's a few other things to consider:
-A speaker that will get you more bass and less highs without breaking the bank (I haven't tried it but I do like the Celestion G12H30 it's "based" on)
The "55hz" spec represents the lower resonant frequency and should result in a more bass-rich tone
https://wgs4.com/Reaper55 (https://wgs4.com/Reaper55)
-I have always raised the value of the entrance cap to the P.I. from the stock .001 (by at least doubling it with a parallel cap of the same value, but different brand/composition, based on experimentation)
-My favorite mod for thickening it up is to replace the MID cap with a much lower value, which brings the slope of the mid-bass EQ up A LOT(I use .001 or .002)
Put it on a switch as a "mid-boost" with the low value cap in the "boost" position and the stock cap as the "OFF" position
-The MID cap trick PLUS the PI entrance cap adjustment gives you a wide range of "tune-ability", but obviously take you away from the stock arrangement.
Best of luck tuning it in
EDIT- I found this video that I feel does a good job of letting us hear the difference in the 55hz nodel
https://www.youtube.com/watch?v=hevJjWLyfWg (https://www.youtube.com/watch?v=hevJjWLyfWg)
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Thanks! Like I said on previous post, will be back in two weeks. See you then. Platefire
BTW-Got a new smart phone. So if I figure it out, might be back sooner on that :dontknow:
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I've been thinking about the higher than normal voltages downstream fromm
Power rail B. Thinking of increasing the 10k between rail B & C upward to see what that does. Platefire
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Got a question about my last check on voltages listed on post #19. On V4 PI plate voltage is 242/220 yet my "C" rail voltage is 325 V just like the Fender DR layout Dwg. Also the plate voltages on the Fender layout is 180/170. OK-----having established that and what I can't resolve in my thinking about it is----if my "C" rail voltage iis 325 same as Fender DR, then why is my PI plate voltages so much higher-----I don't get it?? Patefire
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I don't get it??
Maybe it's because you are using a 12AX7 for the PI rather than a 12AT7.
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Well I just never observed and noted a 12ax tube making that much difference in plate voltage before? This transformer/choke set came out of a brand new DRRI that was designed for a normal channel plus I have a dias based tremolo. That's just one 12ax less than original transformer design, so it shouldn't be that much over powered to make that much difference it seems tolike. Platefire
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Well I just never observed and noted a 12ax tube making that much difference in plate voltage before?
Since it's on your mind why not try it now. Let us know what you find out.
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Well I'm not at home. In NC for anther week. I am working this amp long distance :l2: So it will be another week to see what a 12AT7 would do to the plate voltage. I'll try it for grins when I get back. I know the current draw is different on the two tubes and I suppose that's what's causing the ruckus! Platefire
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Well I'm back now. I tried the 12at7 in the PI and the voltage reading went down to 176/184 as compared to stock Fender 170/180----with modern AC suppy, not bad!
My reverb control is super sensitive where it gets real wet fast. I installed 12au7 in my v2 driver and that helped some. Also v3 and v1 is about 30 to 40 v high. Think I'm going to experiment with increasing main resistor to node D to lower voltage to v1 and v3 iin one sweep.
My schemtic shows v3a voltage from node C incorrectly, it should be and is from node D.
I really would like to hear it at vintage stock voltages before deciding any further changes other than maybe a different speaker. Platefire
BTW---think I'll leave the 12at7 in the PI for now
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My reverb control is super sensitive where it gets real wet fast.
Replace the pot with a 100KA (log taper)pot.
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Thanks Sluckey for the tip on the reverb pot.
Got a turret/de-solder question since I don't do this very often and get a little nervous due to my lack of experience pulling parts instead of installing.
OK---this is the situation---you got two resistors coming into the same turret head with the leads down in the top hole with a nice cap of shinny solder on top. Whats the best method of removal? My thought would be to heat the top of the turret up until the solder starts melting and pull the resistors leads out with needle nose pliers----then re-heat it and suck excess solder out of the hole with a solder sucker. This may not be best? If so please inform me! Platefire
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I usually heat the top of the side of the turret rather then straight down on the hole of the turret. Use pliars & then a solder sucker.
With respect, Tubenit
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Thanks tubenit
I've had a lot of experience on old style p to p but not on turret board de-soldering/removal.
Platefire
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Did a lot of the changes discussed and Man! On first impressions, I think I've got a lot sweeter DR.
Changed the NFB resistor to ground from 100 to 47
Permanently added a 22uf/50V to trem first stage cathode. This improved the intensity nicely.
Changed out power resistor from node C to D from a 10K/3W to a 22K/3W
The 22K resistor to node "D" really brought the V1 & V3 voltages in line with Fender DR AB763 levels within just a few volts. That changed the amp quiet a bit. The clean headroom, overall amp volume and brightness was reduced in one sweep. The amp sound now has more body and less ice picky. The amp is more touch responsive with the tone having more even harmonic content. Voltages across the board are pretty much in line with original DR, so hopefully I'm now getting a reasonable facsimile of what a vintage DR would sound like. I'm still debating with myself about a NFB switch or pot?? Attached is the schematic with I think all the latest changes incorporated. Thanks very much for everyone's help!!! :worthy1: Platefire
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Just wondering??? How many really use their NFB switching/pot arrangement on a regular basis. tubenit said he didn't use his, put it back stock and plugged the hole. Ed said he likes a pot to dial in a sweet spot in a smaller room to get a little more breakup.
I've been thinking about possible versatile arrangements----DPDT Center off--with center off=no feedback, jumper on one set of poles for normal feedback and the other set of poles with a fixed resistor or connected to a pot.
Also I was thinking of a pot with a on and off switch. Run the NFB from speaker to the input term of the pot first, then from pot output term to one term of the switch and from the other switch term to the regular stock NFB resistors of amp. Switch off would be no NFB, switched on would be stock NFB+pot resistance as set and full clockwise would be the stock DR NFB setting.
There is probably better ways to do this that has been rehashed before--these were just some of my thoughts going beyond just a SPST switch with a little more versatility. All this may be overkill, but I did try no NFB and kinda liked it!?!? I thinking for a blues digger like me maybe a nice option. But really---I use pedals for my dirt! so you can see the debate is still raging with myself :icon_biggrin:
sluckey---you recommended a 100kL pot for reverb---I checked and that's what is in there just like the original DR schematic.
I've really been getting into DR's lately. Took my Allen Accomplice to church and set it up. It has a Emi Texas Heat.
Really sounding good with my strats/pedalboard. Tweaking my Peavey DR at homes so I'm DR to the max here lately.
Not a bad place to be! Platefire
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sluckey---you recommended a 100kL pot for reverb---I checked and that's what is in there just like the original DR schematic.
Uh, no. I recommended you REPLACE that pot with a 100KA (log taper).
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OK, sluckey--sorry! you got to cut us old men a little slack :l2: guess I'll have to order one, none in stock!
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Think I've decided what I want to do on NFB. I like Ed's use of a pot for fine tuning the NFB or lack thereof to your taste. Maybe just a little extra resistance to make a smaller room more cozy but----I also like the ability to go full grown Nada NFB. I don't want to drill a lot of holes--one is enough.
So I came up with the idea of using a switchable pot. The only problem I couldn't fine any 100 or 50KL switchable pots. I have a non-switchable 50KA so I measured resistance and going counterclockwise from 4:00 oclock fully open(no resistance) it goes to about 12:00 oclock only gains 5K resistance. So the other 45K is from 12:00 to 8:00 oclock---then switched off---no NFB. I think that will be fine!!! So Doug had a 50KA switchable, so I ordered that. Attached is a quick and dirty sketch of hookup and operation. Platefire
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I like to use a locking pot. I never used a switchable pot as I never found a need. I have used switches for different R values. I don't change them often. I find I change more on Plexi and JCM800 builds.
I don't change them much on my Deluxe or Princeton builds but I like having them, but when you hear a Fender amp with little NFB and add some raw you really kinda sorta have it available. Makes them dirty little critters. Kills the blackface vibe, but I remember listening to Pete Anderson talking about what Steve Fryette modding his 66 Deluxe he did these things. Pete said he recorded all his early stuff with Dwight Yoakam using the Deluxe but mainly using all mids from the raw that was installed.
I have also heard of changing to a cathode driven tonestack. Supposed to be a fairly simple mod to do this.
Have you decided on your speaker or speakers? This past weekend I played the Blackjack 21 FUCHS amp with the Wizard speaker and found out it is supposed to be like the G12-30 Celestion. Basically like a greenback. I really like that combo and if I did not have too many amps I would really like to have one of these.
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Regarding locking pots, no this is not a locking pot but one thing I do sometimes when I don't want a pot to float out of adjustment is put on a type knob with a tension set screw and allow the bottom of the knob to drag against the surface(pickguard or chassis). It will still turn but the drag keeps it from drifting. That's what I had in mind on this or you can just leave the knob off and that will usually allow the setting to stay in place. That setting would be something I would check every time I use it on start up. Anyway this NFB setup would allow me to get three different things (1) fully disconnected NFB (2) adjustable NfB up to 50K (3) Stock DR setting(fully clockwise)
On the speaker, since I've tweaked the amp with lower D rail voltages, installed the 47 NFB resistor to ground, rewired the mid/raw pot, it's been sounding great to me with the existing 25 watt Weber. This Weber was a special design by Gerald W for users of the Silvertone 1482 for folks on the now defunked Vintage Silvertone Forum. I ended up putting a Emi Cannabis Rex in the 1482 and put the Weber in my DIY DR. I haven't dropped the new speaker Idea, but with the amp sounding better, it's less of a priority. The Wizard and the 12C/S are good prospects to try. I've been jamming on it everyday and what a pleasure! Pure Fender tone at its best. Platefire
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Well it is great to hear you you have it tweaked. It is really a great all around amp. I guess when it began to loosen up some. I mean you play 5E3 and such which are greasy feeling and even the JTM45 feels a little like that as well. But modding a blackface where you can get that gritty sound and easily return to a Blackface Fender is what I have always preferred in a club type amp.
I like Weber Speakers and have a lot of them. 10A125 (20watt) and 10F125 (30 watt), 2 twenty watters on top and ceramics on the bottom in s super reverb is the best speaker setup I have every heard.
Thing is I think you are saying the amp is feeling familiar and you are beginning to grow an attachment to it.
KEWL BEANS BOB!
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When I built this amp in 2008 it was such a long drawn out affair planning and making everything fit in that Peavey Pacer Chassis/Cab that I apparently didn't have any tweak left in me when it was finally done. So I eventually begin to realize it wasn't quiet right. I played a Silverface DR in the 70s that sounded great, so I knew it was suppose to be good, even though I never played the Blackface circuit. So here 7 years latter I'm finally getting around to the tweaking :dontknow: and it's now sounding like the DR I remember.
This amp is kind of special though, because it is the last remaining piece of equipment from the old days. The old SS circuit use to rock pretty hard with the OD control activated by a foot switch(like another channel). It was a loud 45 watts. So the new plan was to retain the old Peavey Pacer look but being incognito for a DR lurking underneath. A casual observer would never know. The letters/symbols on the front face of each original control knobs tells what the true function of the pots are in opposition to what the old original face plate indicates. I actually have a legend(see pix) on the back of the chassis that spells out the operation of each knob for who ever ends up with this amp after me. So this amp is so personalized that anybody else would have to study it and figure it out before they could properly operate it. This has to be one of the weirder conversions in guitar amp history :icon_biggrin: ---regarding hidden cosmetic identity not the circuit.
I got the the parts in from Doug today to try a 100KA pot for the reverb and try my switchable pot idea for a NFB control. If those tweaks work out like I hope, that should be it for the circuit. I'm trying to be patient, take my time and do it right this time. Platefire
BTW-I've already got my line worked out what I will say when someone says "That's a fine sounding Peavey" I'll say "yelp, it sure is".
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I've got the amp all put back together. Satisfied that everything is good. I did try the 100KA pot on the reverb and it did give me more travel adjustment but I liked the sound of the 100KL better so I ended up putting it back in. Putting the 22/50 cap on the tremolo cathode livened the intensity up where I am real happy with that. Last but not least, the NFB switchable pot works really well. I took some pixs that is attached. It's a shame I have waited this long(since 2008) to make these changes but as they say "it's now or never". Got one sweet DR now. Thanks for your help! Platefire
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I did try the 100KA pot on the reverb and it did give me more travel adjustment but I liked the sound of the 100KL better so I ended up putting it back in.
Can you describe the difference in sound between the 100KA and the 100KL pots?
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Pfire; would you be up to making the changes on your schemo and posting it? would be nice to see the 'finished' project with your tone tweaks included.
gracias :icon_biggrin:
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On the 100K A and L pots. To me the "A" sounded brighter and some of the previously earned warmth seemed to be missing. Also the reverb sound changed in a way I'm not sure how to describe but was less favorable as I remembered the L pot. I reasoned with myself, this can't be! Just the difference between Liner and Audio taper couldn't do this? After playing on it several times between breaks and coming to the same conclusion every time, I finally decided I may as well change it back or I wouldn't be satisfied. So that's what I did and was happy with the sound when I changed it back. I have no logical explanation why or how it could be that much difference in the pots? Bottom line is you have to follow your own ear and instincts on something like that, even when it don't make sense.
Attached is the revised schematic. De Nada, Platefire
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Bottom line is you have to follow your own ear and instincts on something like that, even when it don't make sense.
Just the fact that your ear can tell a difference is a good sign at any age, and just because it doesn't make sense doesn't mean it ain't better.
Since your an obvious connoisseur of the finer nuances of reverb, I've got a couple tweak ideas that have not been mentioned yet in this thread.
- Change the 500p cap that feeds the input of the reverb driver to .001 to allow a little more mid body into the mix, and maybe sound warmer to your obviously seasoned ears (got that one from tubenit)
- Try removing the bypass cap on the reverb driver 12AT7 to calm down the wetness of the reverb. (from PRR)
If you could hear the difference between an A pot and an L pot, you'll definitely hear these differences.
This mod will give you a wider useable range on your reverb pot, and dampen the overall wetness of the reverb.
Both tweaks together made their way into my last build.
Either way, I've enjoyed hearing the results of your tweakin'
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Just the difference between Liner and Audio taper couldn't do this?
I'd measure both those pots for resistance. Maybe their way off from 100K?
It might be harder to dial in the verb with A taper but brightness/warmth difference seems odd.
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Thanks for the tips on the Reverb but truly I'm really happy with the reverb sound. The only thing that was bothering me is that I was getting my reverb sound at 1.5 where normally I have to turn up to about 3 or more. So no problem with the reverb sound. The short amount of travel conflicted with my previous fender type reverb settings experience. That's what sluckey was trying to help me by suggesting using the A pot---more travel to get the same thing but the problem is it didn't sound the same to me and I kept missing my previous sound. So I had to go back and get it! So I'm totally happy with my reverb sound, so no need for tweaking any further. I can re-calibrate my fender reverb experience for shorter travel tweak on this amp.
The truth is I'm loving the sound and feel of this amp now so much that I don't want to do anything to interfere, disturb or change it---if you know what I mean! I've got the amp buttoned up and can't see no reason at this point to tear it down and resume tweaking. I've noticed it's even quieter regarding operation noise also and I love a quiet amp. So I ask for help on tweaking this amp and all of you came through and pointed me in the right direction. You know previously I wouldn't even play this amp because I just wasn't happy with it and avoided it. So now I'm very pleased with it and certainly won't be avoiding it anymore. I thank you ALL very much for helping me get there!! :worthy1: Platefire
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The truth is I'm loving the sound and feel of this amp now so much that I don't want to do anything to interfere, disturb or change it---if you know what I mean!
Oh, I know what you mean....I felt the same way after I built a cathode biased version of sluckey's TDR and tweaked it to perfection....
Only problem was, I built it for someone else and hated to give it up. There really is something to that circuit.
:thumbsup:
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Well!!! It's time to make a TDR for yourself---and if someone buys that, then another one :laugh:
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I made a removable panel to make it a closed back cab. The two top keepers rotate to hold the top in place securely.
I'm just wondering if the opening on the top back plate panel at chassis is enough to let the tubes breath sufficiently? Just finished it, so I will jam a little to see what it sounds like. I might aught to get my infra-red thermometer out and watch the temperature on the tubes. Platefire
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I'm really liking the sound of the amp with the added back panel on. Best I can tell added a measure of bass, little louder and little more responsive is the effect I'm getting---for bedroom jamming adds up to additional layer of sweetness--and that is good. Not sure how that would pan out in live band situation? The rectifier and 6V6S's are running around 190F in the hottest spot---that's not bad is it? The added back panel is not air tight--has openings for power cord, trem/reverb cord plus a small 1/16 crack at top where it joins the bottom of the top back plate.
If this added panel works out, I'm thinking of added one to my Allen Accomplice. Platefire
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I picked up a small 4" fan at Wally World today that is 120VAC with a regular power cord. I thought by just looking at the swivel stand in the store that It should be able to hang on the upper back panel. When I got it home and tried it, sure enough it hangs fine. So at the least it's a good temporary solution to restricted tube ventilation when the closed back panel is installed.
EDIT: haft to say I've been trying the amp with the closed back panel on and off. With the strat I'm using sounds much better with it on. The response/touch feel/sustain is much improved at the same volume/tone settings with it on and just a little warmer tone. The fan is doing an OK job of keeping the power tubes and rectifier cool. I was a little concerned the fan would effect the speaker with a fluttery sound but best I can tell it's not. I'm thinking that the close back panel is a + for the DR sound. :icon_biggrin:
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I know it's time for this thread to go away but----took the amp to the Church, set it up to play with the Praise Band and used it this weekend. I wasn't disappointed. I used my tele, the brightest worst case scenario for brightness and ice picks. I was able to set the amp to mellow it down to where it was a sweet tele. If this amp can smooth out a tele all my other guitars will be a cakewalk. I never trust my own bedroom amp assessments much until I put it through a live band playing situation that to me is where the rubber meets the road. Passed with flying colors! I'm a happy camper---Thanks, Platefire
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Another happy ending. :grin:
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Sorry! Had to add one last finishing touch to this documentation. I just labeled my NFB control and added the pixs of it in it's normal AB763 Blackface position and NFB off position. Everybody can go back to sleep now :l2:
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Inspirational!
How did you do the lettering? I need to do some back panel lettering on my next amp. I've done dry transfer letters, one at a time. I was not been happy with my spacing. Yours looks so professional!
Craig
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You can get sheets of dry transfers that have common audio and electronics words on them. Much easier than individual alpha/numeric sheets. Platefire used a label printer with black text on clear tape. Dymo and P-Touch are two brands I've used.
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I use a Brother P-Touch with clear tape. Looks like this.