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Post subject: Are the new screen resistors sucking the tone out of our amp
Posted: Sat Jun 16, 2012 7:04 pm
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On another forum I have already described how my son & I increased the volume & livened up the tone of our Deluxe Reverb II (DRII) by swapping valves (tubes). http://www.fenderforum.com/forum.html?d ... 2322:17:24 & http://www.fenderforum.com/forum.html?d ... 0222:53:44
If you’ve already responded to this question on the Fender Discussion Page Forum, don’t feel obliged to do so here unless you want to.

In spite of the fact that this amp has given 30 years of essentially trouble free service, a little reading on the net (a little knowledge is a dangerous thing) convinced me that we should attempt to ensure the reliability & convenience of this amp by three simple mods.

1. Incorporate an adjustable bias pot such as is on the DRRI as per the instructions on stratopastor’s site. I managed to find a local site that sold authentic Fender bias pots & this mod has certainly increased the convenience of doing bias adjustments when swapping power valves.

2. Exchange the ½watt 47Ω bias feed resistor for a 1watt 47Ω one to hopefully increase reliability & head off cascading disasters if anything should go wrong earlier in the circuit.

3. Insert 470Ω 1 watt metal oxide resistors on the 6V6 screen grids as are on just about every other Fender designed amp. Paul Rivera used them when he designed the Concert II & the Twin Reverb II but not on the Champ II, Super Champ, Princeton Reverb II or Deluxe Reverb II.

Having performed these mods & re-biased the amp to take account of the circuit changes we were disappointed to find that the volume & tone had been diminished. We also have a Deluxe Reverb Re-Issue (DRRI) & before these mods, we felt that the DRII was better in volume & tone than our DRRI but now the bass had certainly been sucked out of the DRII & it had to be played at a notch higher in volume (i.e. DRRI @ 3 = DRII @ 4) to be at a similarly perceived volume.

More reading on the net, particularly from http://www.thegearpage.net/board/archiv ... 74440.html regarding screen grid resistors revealed comments such as “if you go too high on the screen grid resistor it can change the tone...to me it reduces punch and taken too far can make the amp sound anaemic/constipated”, “noticeably less punchy/alive”, “Too high a value on the screen grid resistor definitely chokes out the tube”. Now it doesn’t seem to me that 470Ω is too high a value as it is after all the value that Fender generally uses but it does seem that our amp is now suffering these effects.

Because of its utility I hope the new bias pot is not the cause & I can’t see how changing the wattage of the bias feed resistor could have such an effect because it’s the same value of resistance. The reduction in volume & tone seems to come down to using screen grid resistors in this amp so maybe there was a reason that Paul Rivera left them out.

Does anybody have any experience or comment about this?


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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Sat Jun 16, 2012 8:30 pm
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Short answer, yes--- any resistor in the signal path is going to change the tone and response of the amp. Keeping each resistance as low as possible, and still do the work it was designed to do, is the key.

The function of a screen resistor is to insure that the screen is at a lower VDC than the anode. This helps keep the output tube from thermal runaway. 470-ohms drops the voltage about 5 VDC from the plate (for ~400-450VDC). This seems to be a good range, which doesn't effect the tube's transient response too much. Marshalls often run 1K-ohm (about 10VDC drop) on their EL34's.

If you have very precious tubes (like original 1940's TS "coke bottle" 6L6G/VT115A's)... you may want to use 1K-ohm WW resistors on the sceens. Good wire-wound is the best, in terms of precision, heat-resistance, and the least amount of inherent inductance.


Last edited by BMW2002Ti on Sat Jun 16, 2012 10:40 pm, edited 1 time in total.

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Sat Jun 16, 2012 10:24 pm
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Resistors on the Plates?

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Sat Jun 16, 2012 10:39 pm
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Shimmy, of course you'll have dropping resistors going to the anode plate. But, I was talking about the resistors for the screen grid.

EDIT: I corrected my error, in my original response. Thanks!

Like the 22-ohm ones, on the 6550 of the SVT amp.

http://www.drtube.com/schematics/ampeg/svt6550-jp.gif


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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Sun Jun 17, 2012 9:34 am
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I thought that was what you meant, but remembered you had also mentioned Plate resistors for output tubes before. Interesting that the SVT has low value resistors on the output tube Plates, and fairly low value screen grid resistors. I had one recently (6550), the Plate resistors didn't come to mind until I looked at the schematic again, but I sure remember the weight. :o

Have you done much in the way of adding Plate resistors where none existed before, especially as far as tone comparison? I would expect some noticeable affect, as the Plate voltage will change inversely with current changes through the resistor. Do you feel that a Plate resistor works well for protection when using high Plate voltage, in the same way that a screen grid resistor works to limit the screen voltage? Or other reason?

I have changed value of screen grid resistors, and noticed some pleasing compression in the sound as the value is increased, but have done nothing with Plate resistors. Increasing the screen grid resistor value will approach triode operation, I can see how that affects tone and output power.

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Sun Jun 17, 2012 10:11 am
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Shimmy, I haven't done much experimenting with plate resistors on output tubes. BUT, have played a lot with gain stage (esp input stage) resistance and isolation 'lytic caps to get a browner tone. Which is better isolated from other stages. Specific vales of resistance and caps depend on the overall gain desired and the tube types used in this critical stage. And other time-constant issues.

BTW... using a 1-watt metal oxide resistor is a great idea for the grid stopper and screen resistor on any 6V6GT or 6L6GC amp. Because Fender puts them right on the socket. You need heat resistant resistors, for long-term reliability. 1-watt rated will have much better heat tolerance than 1/2-watt (everything else being the same). It's why I like WW resistors (best heat tolerance) or precision 1% Takman or Riken resistors.

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Sun Jun 17, 2012 10:25 am
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Shimmy, BTW... when you have several output stages in parallel, like the SVT, there seems to be a school of thought that the accumulated effect of several screen and anode resistors in parallel can have an adverse effect on tone and response. At least Ampeg bought into this. So they tried to use the least resistance possible. And the best tubes like TS 6550 or GE 6550A. And not even the famous GEC/MOV KT-88.

When redoing this amp, to insure stable life with this arrangement, 1% WW resistors, like Mills, are a must. As catastrophic failure can happen to the entire stage, if one push-pull section goes unstable. Common with modern "6550" tubes.


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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Thu Jun 21, 2012 4:52 am
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Tried jumping the screen resistors with wires that had alligator clips on both ends and there was no improvement in tone – bummer! Really didn’t want to take them out & really didn’t want to lose the other “improvements”.

The only other thing to try before more surgery was valve substitutions. You may or may not recall from my “Adventures in valve testing” topic from the Fender Discussion Page Forum how we greatly improved the tone of this amp by plugging in used old stock Australian Miniwatt 12AX7’s in positions V1, V5 & V6 instead of the old USA made Fender Specials that had given 30 years of sterling service.

When we made these reliability modifications over a week ago now, we removed all the valves & when the work was done we hurriedly put them back in because the amp was needed for teaching purposes & students were arriving. While we did put the valves back so that the same type & brand was in the correct position, we didn’t necessarily put the same Miniwatt into V1 that had been there before, or even the Fender Specials from V2 & V4 the same way round as they were before.

By now, we had also bought a set of Tung-Sol 12AX7’s & JAN-Philips 12AT7’s which had gone into our DRRI so we pulled them all out of there & put them into the DRII & Lo & Behold, the tone came back.

From there, we split up the valves we had between the two amps so that the better valves giving the best tone were in the critical positions of both amps & now they both sound great. Let me know if you want to know what type of valve in what position we’ve settled on.

So thank you to all who gave their advice & commiserations for this project – it seems that those who held out for the problem being something other than the new mods were on the money.


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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Thu Jun 21, 2012 10:28 am
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AnthonyW wrote:
...Let me know if you want to know what type of valve in what position we’ve settled on...


That would be nice. :)

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Thu Jun 21, 2012 4:37 pm
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Original valves in amp when bought:-

Deluxe Reverb II
V1 Fender SPECIAL 12AX7/A (USA)
V2 Fender SPECIAL 12AX7/A (USA)
V3 unknown manufacturer 12AT7 (China)
V4 Fender SPECIAL 12AX7/A (USA)
V5 Fender SPECIAL 12AX7/A (USA)
V6 unknown manufacturer 12AX7 (Russia)
V7 Fender SPECIAL 6V6/GTA blackplate (USA)
V8 Fender SPECIAL 6V6/GTA blackplate (USA)

Deluxe Reverb Re-Issue
V1 Fender Groove Tubes Sovtek 12AX7/WC (Russia)
V2 Fender Groove Tubes Sovtek 12AX7/WC (Russia)
V3 Fender Groove Tube 12AT7 (unknown country of origin - probably Russia)
V4 Fender Groove Tubes Sovtek 12AX7/WC (Russia)
V5 Fender Groove Tubes Sovtek 12AX7/WC (Russia)
V6 Fender Groove Tube 12AT7 (unknown country of origin - probably Russia)
V7 Fender Groove Tubes Electro-Harmonix 6V6 (Russia)
V8 Fender Groove Tubes Electro-Harmonix 6V6 (Russia)
V9 Ruby Tubes 5AR4 (unknown country of origin - probably Russia)

1st change for improved tone before mods:-

Deluxe Reverb II ( using our only 3 used old stock Miniwatts)
V1 Miniwatt 12AX7 (Australia)
V2 Fender SPECIAL 12AX7/A (USA)
V3 unknown manufacturer 12AT7 (China)
V4 Fender SPECIAL 12AX7/A (USA)
V5 Miniwatt 12AX7 (Australia)
V6 Miniwatt 12AX7 (Australia)
V7 Fender SPECIAL 6V6/GTA blackplate (USA)
V8 Fender SPECIAL 6V6/GTA blackplate (USA)

Deluxe Reverb Re-Issue (using all our newly acquitted pre-amp valves)
V1 Tung-Sol 12AX7 (Russia)
V2 Tung-Sol 12AX7 (Russia)
V3 JAN-Philips 12AT7/WC (USA)
V4 Tung-Sol 12AX7 (Russia)
V5 Tung-Sol 12AX7 (Russia)
V6 JAN-Philips 12AT7/WC (USA)
V7 Fender Groove Tubes Electro-Harmonix 6V6 (Russia)
V8 Fender Groove Tubes Electro-Harmonix 6V6 (Russia)
V9 Ruby Tubes 5AR4 (unknown country of origin - probably Russia)

2nd change for improved tone after mods (to spread good valves between both amps into critical positions):-

Deluxe Reverb II
V1 Miniwatt 12AX7 (Australia)
V2 Tung-Sol 12AX7 (Russia)
V3 JAN-Philips 12AT7/WC (USA)
V4 Fender Groove Tubes Sovtek 12AX7/WC (Russia)
V5 Miniwatt 12AX7 (Australia)
V6 Tung-Sol 12AX7 (Russia)
V7 Fender SPECIAL 6V6/GTA blackplate (USA)
V8 Fender SPECIAL 6V6/GTA blackplate (USA)

Deluxe Reverb Re-Issue
V1 Miniwatt 12AX7 (Australia)
V2 Tung-Sol 12AX7 (Russia)
V3 Fender Groove Tube 12AT7 (unknown country of origin - probably Russia)
V4 Tung-Sol 12AX7 (Russia)
V5 Tung-Sol 12AX7 (Russia)
V6 JAN-Philips 12AT7/WC (USA)
V7 Fender Groove Tubes Electro-Harmonix 6V6 (Russia)
V8 Fender Groove Tubes Electro-Harmonix 6V6 (Russia)
V9 Ruby Tubes 5AR4 (unknown country of origin - probably Russia)

Even though for V7 & V8 we have a mixed bag of 5 tested viable used old stock (PHILIPS/Miniwatt, AWA Radiotron, Brimar) & 2 brand new JJ 6V6’s, the existing power valves are giving a great result & after our recent experience, we’ll probably leave well enough alone for the moment although I imagine that in the future, we’ll get itchy to try them out as well.

I continue to be amazed at how the 30 year old Fender SPECIAL 6V6/GTA (USA) & 50 year old Miniwatt (Australia) 12AX7 valves soldier on giving great tone superior to more recent brand new valves.


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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Mon Dec 10, 2012 9:59 am
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Too high of a resistor obviously affects current levels which in turn change the tone. Can I ask what sort of power supply are you using with this setup?

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Mon Dec 10, 2012 1:51 pm
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The power supplies for both these amps are stock standard originals as supplied by Fender for our local electricity supply – 240 volts/50 cycles per second NOT the North American 110v/60cps – but that voltage difference is unlikely to have an effect on tone as either type feeds their respective circuits with the expected correct voltages.

In the end we found that the modifications had no perceived effect on tone & that the difference was all down to what valves were in which positions in the amps.

In summary, the mods we made to these amps were:-
For the Deluxe Reverb II:-
1. adjustable bias pot
2. 1watt 47Ω bias feed resistor
3. 470Ω 1 watt metal oxide screen grid resistors
For the Deluxe Reverb Re-Issue:-
1. 1 wire Fritz mod that allows both channels to have reverb & vibrato
2. 0.01 uF @ 1KV ceramic disk capacitor to stop ticking from the vibrato section (but to R43 instead of R45 – refer to http://www.fenderforum.com/forum.html?d ... ber=775553)


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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Mon Dec 10, 2012 1:55 pm
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Hi pavel87,

See the schematics linked below.

Deluxe Reverb Reissue

Deluxe Reverb II

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Mon Dec 10, 2012 11:20 pm
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As little tidbit... since the AC signal rides on top of the DC applied onto the anode plate, the plate loaded resistor (100k-ohm in this case) can effect the overall signal or tone.

The AC waveform is somewhat effected by the 200VDC rail voltage. You can tell because when the lytic cap (20mfd/450VDC in this case) fails... 120Hz is fed into the signal. Good PS has to isolate sections from sections.

Grid resistor (grid-to ground) effect the idle bias of the tube. And has direct effect on tone. Screen resistors effect electron flow through the output tube. Or any pentode in the circuit. Too much resistance on the screen = bigger difference between screen and plate voltage. Choking the signal path and directly effecting output power.

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Post subject: Re: Are the new screen resistors sucking the tone out of our
Posted: Tue Dec 11, 2012 11:23 am
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BMW2002Ti wrote:
...The AC waveform is somewhat effected by the 200VDC rail voltage....


To be more specific, referring to the half of a preamp tube shown, the 200 VDC on the Plate is at idle, and this voltage rises and falls inversely with the raising and lowering of the current drawn through the tube, controlled by the AC signal into the tube's grid. The voltage at the Plate is 200 VDC at idle, and the more current drawn by the tube through the 100K Plate resistor, the higher the voltage dropped across the Plate resistor (lower Plate voltage), and the less current drawn by the tube, less voltage dropped across the Plate resistor (higher Plate voltage). The AC signal waveform into the tube grid controls the rising and falling of the Plate voltage because it controls the amount of current through the tube and thus the current through the Plate resistor which determines the voltage drop across the resistor. This rising and falling DC voltage drop across the Plate resistor is what becomes the AC voltage that is fed to the next stage, with the coupling cap only allowing the AC to pass, effectively blocking the DC. Repeat the process in the next stage (next half of a preamp tube) if there is one, amplifying the signal at each stage, with each stage outputting a higher signal than the last. More stages, more gain, higher signal voltage to be fed to the PI and output tubes.

This is very similar to what happens with the output tubes, except that in the case of the output tubes, the current is controlled through the OT primary instead of through a Plate resistor, and the current rising and falling through the output tube and the OT primary becomes the AC signal to the speaker from the OT secondary.

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