Crate V5 mods... anyone?

[jim p]

I am a little confused on your post you talk about R17 and the triode at the same time R17 is the cathode resistor for the EL84. I am trying to check what I had as a final value and it looks like 270 ohms, but it is best to use a voltmeter and check plate voltage and cathode voltage to calculate the power dissipated by the tube when making these changes. Not sure I covered amplifier output tube bias in this thread if you go to Sewatt on the Valve Jr page you can find it there. If you do parallel R17 to get say 270 ohms with a 1.5k the 1.5k can be a 1 to 2 watt resistor.
On the bass response end one big limit to the stock bass response of the amplifier was the capacitor values used in the op amp circuit, you now have that bypassed. I know I posted what I found for bass response and it is as good as it gets on this amp especially with the 20:1 turns ratio and an 8 ohm speaker. So even if you changed the output transformer I doubt you would do much better.

[rjohnj]

I am a little confused on your post you talk about R17 and the triode at the same time R17 is the cathode resistor for the EL84. I am trying to check what I had as a final value and it looks like 270 ohms, but it is best to use a voltmeter and check plate voltage and cathode voltage to calculate the power dissipated by the tube when making these changes. Not sure I covered amplifier output tube bias in this thread if you go to Sewatt on the Valve Jr page you can find it there. If you do parallel R17 to get say 270 ohms with a 1.5k the 1.5k can be a 1 to 2 watt resistor.
On the bass response end one big limit to the stock bass response of the amplifier was the capacitor values used in the op amp circuit, you now have that bypassed. I know I posted what I found for bass response and it is as good as it gets on this amp especially with the 20:1 turns ratio and an 8 ohm speaker. So even if you changed the output transformer I doubt you would do much better.

sorry, i've probably confused you a little because i'm still learning all this.... the past week has been a steep learning curve for me. I will definitely try to learn the theory in the next few days for the most appropriate way to deal with the the bias. (I have a bias measurement tool on the way as well).
thx

[schenkadere]

Aside from the obvious speaker change and tube change, which I did with a Warehouse Veteren and JJ's, what is the simplest mod to get this amp sounding even better? And...do any of you guys do this work? I'm a soldering spaz.

[jim p]

Look at post #168 this takes the op amp out for an all tube amp.

[rjohnj]

Look at post #168 this takes the op amp out for an all tube amp.

I've completed all my mods and the amp sounds great - better than anything else in it's class, except for one thing..... when I have the amp and guitar cranked and hit the E-string or power chord (especially on the neck pickup) the amp gets really 'farty'.
I've messed with the bias and measured all my voltages and if I keep it in the 'healthy' range it still gets farty. I've tried running it really hot and it seems to help but then the power tube red-plates. The standard 330 ohm cathode bias resistor actually seems to be about right to keep the amp within the recommended limits - it's even at the upper end of acceptable.Though I'm currently a touch hotter than that to help with the fartiness - 14v plate dissipation. Any recommendations on how I can solve this or do I just need to live with it?

Ryan

[jim p]

Just to check on your plate power dissipation calculations. You should have determined the cathode current then subtracted the screen grid current to obtain the plate current. Then the voltage across the tube which is plate voltage minus the cathode voltage is the voltage across the plate times the plate current for the power dissipation of the tube. The ball park number screen grid current is 4mA but there is a screen grid resistor you can measure the voltage across to get an exact number if you want.

No answer on the farty sound I would try driving another speaker if one is available to use.

[rjohnj]

Just to check on your plate power dissipation calculations. You should have determined the cathode current then subtracted the screen grid current to obtain the plate current. Then the voltage across the tube which is plate voltage minus the cathode voltage is the voltage across the plate times the plate current for the power dissipation of the tube. The ball park number screen grid current is 4mA but there is a screen grid resistor you can measure the voltage across to get an exact number if you want.

No answer on the farty sound I would try driving another speaker if one is available to use.

without rechecking, i think, I had something like 319 volts between plate and cathode (taken from pin 7 to 3), 11 volts for the cathode resistor. That's with 250 ohms resistance. (screen is estimation).
then i plugged my numbers into the tool below to get my plate dissipation (and to save me doing the sums)
http://www.webervst.com/tubes1/calcbias.htm
I might try plugging through my 1x12 cab and see what happens.

[jim p]

It looks like the Weber calculator allows just 2mA for the screen maybe just being conservative. Going by the 11 volts on the cathode if you had a 270 ohm resistor and the voltage stays at 11 volts your cathode current would be 41ma so would be just about bang on 12 watts on the plate ( 319 volts times 38mA = 12.1 watts)

[rjohnj]

It looks like the Weber calculator allows just 2mA for the screen maybe just being conservative. Going by the 11 volts on the cathode if you had a 270 ohm resistor and the voltage stays at 11 volts your cathode current would be 41ma so would be just about bang on 12 watts on the plate ( 319 volts times 38mA = 12.1 watts)

yeah that's pretty much what I got just pushing the boundaries slightly with 250 ohms to help with the sound a little.
Not sure why I have to run it so hot to get rid of the 'farting' though. 100ohm resistor sounded good but just ran the tube too hot.

[SciHi]

...get rid of the 'farting' ...

I have found that the v1a cathode bypass cap and/or first coupling cap control the flabby bass. For HBs I have used either 0.68 or 1 uF cathode bypass caps, you can go a little higher with single coils, ~2uF. Or use a real small coupling cap, like the vj fender/voxy mod value, 0.0022uF, my v5 has a 0.0047uF coupling cap. I am using 10K R1, added a master volume with 6.8K grid resistor after the mv, and did the screen resistor mod to the el84. My tone stack sucks lots of gain so the amp is very clean and very tight bass but still retains the thick tone. I also tried adding the nfb but it was too thin with my setup. My speaker is also a thick cone Celestion Tube 10 (G10E-30), and doesn't breakup even playing a bass through the amp. I suggest you try to figure out if it is the amp or speaker that is causing the "farting."

Rock_Mumbles posted this on his tech site "zaphod_phil Quote:
I think the moral of the story is to either use a smallish cathode cap (below 100uF) and bias fairly hot with a low cathode resistor value, or use a very large cathode cap (1000uF to 2200uF) and bias cooler with a larger resistor value. In between values will usually sound woofy."

found here:
http://www.18watt.com/modules.php?na...r=asc&start=15

So maybe as you change the el84 cathode resistor value, you need to change the cap value?

[jim p]

One thing to keep in mind as you lower the value of coupling caps and bypass caps you are reducing the bass response. So you may just be eliminating the problem by having less bass, ok if the bass response is not important to you. One thing to search out on the web for is blocking distortion you do reduce this by reducing the value of coupling caps but you can also reduce the value of the grid resistor on the control grid of the tube to reduce it.
When you bias hot by reducing the cathode resistor value what may be the reason for better response is the RC time constant (resistance times capacitance) of the cathode bypass capacitor and cathode resistor is reduced. The cathode of the output tube shifts its DC value when driven hard and the recovery time could be a function of the bypass capacitor used. So instead of biasing the tube hot maybe you should try changing C9 to a lower value by calculation you could go as low as 10uf with a cathode resistor value of 270 ohms.

[rjohnj]

One thing to keep in mind as you lower the value of coupling caps and bypass caps you are reducing the bass response. So you may just be eliminating the problem by having less bass, ok if the bass response is not important to you. One thing to search out on the web for is blocking distortion you do reduce this by reducing the value of coupling caps but you can also reduce the value of the grid resistor on the control grid of the tube to reduce it.
When you bias hot by reducing the cathode resistor value what may be the reason for better response is the RC time constant (resistance times capacitance) of the cathode bypass capacitor and cathode resistor is reduced. The cathode of the output tube shifts its DC value when driven hard and the recovery time could be a function of the bypass capacitor used. So instead of biasing the tube hot maybe you should try changing C9 to a lower value by calculation you could go as low as 10uf with a cathode resistor value of 270 ohms.

great... I'll give that a go!
thanks
Ryan

[rjohnj]

One thing to keep in mind as you lower the value of coupling caps and bypass caps you are reducing the bass response. So you may just be eliminating the problem by having less bass, ok if the bass response is not important to you. One thing to search out on the web for is blocking distortion you do reduce this by reducing the value of coupling caps but you can also reduce the value of the grid resistor on the control grid of the tube to reduce it.
When you bias hot by reducing the cathode resistor value what may be the reason for better response is the RC time constant (resistance times capacitance) of the cathode bypass capacitor and cathode resistor is reduced. The cathode of the output tube shifts its DC value when driven hard and the recovery time could be a function of the bypass capacitor used. So instead of biasing the tube hot maybe you should try changing C9 to a lower value by calculation you could go as low as 10uf with a cathode resistor value of 270 ohms.

Well it looks like changing the bypass capacitor does do the trick. I didn't have a heap of caps at hand but tried a 33uf. Actually seemed worse but then switched to a 3.3uf and guess what, flabby bottom end gone!
Is 3.3uf too low? I still plan to experiment and I haven't yet checked my voltages etc... I don't know what impact this would have?
Also there was a 100volt cap in there and I only have a 50volt - is this okay here?
Thanks Jim and SciHi for the tips on this.

[jim p]

If you have 3.3uf across the 270 ohm cathode resistor that would be a bit low in value equal to -3db at 180Hz. Using 10uf will be -3db at 60 Hz. As far as voltage rating the voltage across the cathode is approx 12 volts max so a voltage rating of 20 volts or more is fine. The bypassing of the cathode resistor on a pentode probably has no big effect on the gain though so it may be no big deal. I need to look at this more to make sure but it is not the same as a triode.

Just checked and the cathode impedance of the EL84 is approx 88 ohms so not bypassing the 270 ohm resistor will drop the gain by about 75%

[rjohnj]

hi again all

Now that I am pretty happy with my amp I am just wondering about a couple of tweaks.
Can anyone tell me if I change R1 to a 1megohm (R2 is removed) if I will expect an increase in signal -and what impact would this have on gain?

What impact does changing the values of c3 or c4 have - can this help further tighten up the bottom end?

thx

Ryan

[chris2002rock]

The good news is that it looks like there are no SMDs on the board.

One thing I would check is the bias. If the schematic I have is correct, the cathode bias resistor is only 100 ohms. It seems like it should be bigger.

You will want to check the plate voltage as well.

What are you trying to accomplish by modding your amp?

tung

Where did you find a schematic for this amp?

FWIW, I am getting to the point where I might just replace the whole schematic with a hand-wired replacement. If possible, I wanted to just replace the tone stack (I doubt the solid state components are entirely to blame)...but I also believe the guy who said he replaced the entire circuit (save the tubes and transformers).

I was hoping for a glance at the schematic before I rip the guts out).

Thanks in advance for any help on this.

[SciHi]

Where did you find a schematic for this amp?

Right here: http://www.thefret.net/showpost.php?...2&postcount=28

[SciHi]

Can anyone tell me if I change R1 to a 1megohm (R2 is removed) if I will expect an increase in signal -and what impact would this have on gain?

What impact does changing the values of c3 or c4 have - can this help further tighten up the bottom end?

So what else did you do? I put R11 (1M) across the input jack, removed R10 and everything between it and R1 (including R2), so R1 is the input grid resistor which should be in the range of 68K - 10K in most peoples opinion. A 1 M should reduce signal.

I removed C3 but also moved the grid resistor (R31) after R16. C3 should drain some highs. C4 is part of the strange tone control, I reworked the tone control to a Marshall 18 watt normal channel tone control.

[rjohnj]

So what else did you do? I put R11 (1M) across the input jack, removed R10 and everything between it and R1 (including R2), so R1 is the input grid resistor which should be in the range of 68K - 10K in most peoples opinion. A 1 M should reduce signal.

I removed C3 but also moved the grid resistor (R31) after R16. C3 should drain some highs. C4 is part of the strange tone control, I reworked the tone control to a Marshall 18 watt normal channel tone control.

I've bypassed the opamp (jumpered d10 to r2/c25 junction to get to pin 7, top of vol pot to junction to junction of r15, r29 and c28, middle of vol pot to r27 to get to pin 2 and bottom of pot to grnd)
changed c1 to 1uf - after trying many values. Helps control the farty bottom end without getting too thin.
pulled out r2, r7, r15, d9, d10, r28, c27.
cathode resistor is 270ohm and bypass cap is 0.47uf - also best after trying many to help control flab in bass.
cut board at c27 to r28
Tried pulling C5 and lowering value but got to much treble.
Amp voltages are all okay and amp sounds quite marshall-ish (or more-so than a fender) and overall pretty nice. Break-up is not over the top but I was hoping for it to be a little louder. If it's on 5 it's a nice clean tone but too quiet.
speaker is now an eminence ragin cajun and tubes are jj's (at7 instead of ax7)

I've just put in the 1M resistor at r1 to try - i think it works?.. sounds fuller maybe?? it's definitely not louder which I was originally hoping for but not much quieter either, if any. Shoud I drop this back down?

I don't mind the tone control to add a little versatility.

what do your mods between input jack and r1 do?
any other suggestions??? louder and more sustain would be good. (nb.I know it's only 5watts)