Rob's 6V6 JCM800

[Markbou]

Hello All,

I hope all is well for everyone.

I want to build Rob's 6v6/EL34 25 watt amp to try and create a Marshall style amp enabling me to bring the amp for jamming out with others.  The 50 watt JCM Lead weighs nearly 80 pounds, so this new amp is intended to make moving my equipment easier.

I'm looking for recommendation for the choke, PT, and OT transformers.  The links to the transformers on build the page for that amp lead to AllenAmps.com which is not selling transformers now due to supply chain issues.

Ideally, I would like a PT with a 50vct for the bias section.  The rest I await your golden advice.

PT:  290/330 ct, 6.3v, and 50v bias lead
OT: 7k:8 with 4,8, & 16 ohm taps
Choke: 4 or 5 Henries at 120mA

I spent most of today looking around for these items and still don't have much to go on, so any advice would be greatly appreciated.

Regards,
Mark

[sluckey]

Easy peasy. Just get a set of iron for a Fender Deluxe Reverb. Available everywhere.

[Markbou]

Thanks so much!

All the stress from earlier is gone now.

I appreciate it.

~Mark

[Markbou]

I went ahead and built the amp with a 330v CT transformer and my B+ is very high.  Super-high, really.

I am getting 474 volts on the EL34 plates.  I remembered at some point that using solid-state rectification results in higher B+, so that is part of my issue, but the 474 seems like 100 to 150v too high for a pair of EL34s when attempting to limit them to 25 total watts output.  And, I am getting around 230v on the pre-amp tubes which is also about 100v too high for the circuits referenced on Rob's page for this amp.

My main question is more of a poll.

If you found yourself in my position, what fix would you apply to bring the voltage down to reasonable levels:

A.) High-wattage Zener(s) connected to the PT CT to ground.

B.) MOSFET/Zener combo

C.)  Use a different power transformer.

Also, I am using a Hammond 290BX (120V @ 60 Hz - 660Vct @ 138 mA - 50V Bias - 6.3V @ 3A - 5V @ 3A.)  If I disconnect the 50v bias tap and just run a wire connected to the B+ to the bias circuit input, would that drop the B+ at all?  I could cannibalize a transformer used in one of my previous 18 watt builds, but if  tapping the B+1 to provide power to the bias circuit lowers the voltage I would be worried about not being able to hit a full 25 watts.

Would it be safe to increase the first dropping resistor to knock off additional voltage.  I imagine there is a limit to the values you can used depending on your cap values and voltage passing through.

And, without using another transformer, in your opinion, would employing a 50 watt, 50 volt zener, plus bias tapping off of B+1, and possibly a high-watt resistor in there somewhere in the B+ line combine well to do the job? 

I play a lot of high-gain lead and I created this amp to be more mobile as moving the 50 watt combo around is a lot of work.  The newly built amp puts out sound as it is but you have to hit a chord fairly hard to get it to play out of the amp, then it cuts off as the volume fades.  Plus I can only get the amp to play sound via the high input; the low input barely registers.  I can only assume that is due to the very high voltages in the circuit.  So far no humming or squealing so the dressing is good, just need to tackle the voltage issue now.

Because I want the silicon rectification for punchiness and response, adding a tube rectifier is not in the cards for this amp, so my guess is the recommendation would be to use a different PT, but I was trying to avoid that if there was another way.

For anyone reading or responding to this, THANK YOU very much in advance for any insight you can provide!

~Mark

[stratomaster]

Don't assume your problem is due to voltages.  Measure and confirm.  Do you have a scope or an audio probe that you can use to trace the signal from input jack to power tube grid?

Given this is a new build you need to scrutinize everything.  Note that the voltages are too high for your liking, but don't assume that is the cause.  Add it to the list of problems to solve.  I think you'll find the cause of your strange cutoff behavior has little to do with the PT voltage. 

[Markbou]

I understand what you are saying and agree.  However, ...

I included the info about the playability for reference in case it helped anyone.   Until I solve the high voltage issue, everything else is secondary to me.  This is my 12th build but the last one was several years ago, hence why I forgot about the solid state voltage drop versus the tube rectifier voltage drops.

You didn't answer the poll question.  What would you do in my position to lower the B+ voltage given the circumstances?

[Ronquest]

C.)  Use a different power transformer.

This is your answer if you want lower B+.

[Ronquest]

What output transformer are you running?

[labb]

Most likely you have a wiring problem.

[glass54]

Hi Mark,

If I disconnect the 50v bias tap and just run a wire connected to the B+ to the bias circuit input,

No!! Bias only draws a couple mA. Completely useless exercise and a dedicated Bias supply winding is a bonus. Stable Bias supply is an excellent asset.
Do you have a current sense resistor on you EL34's ie 1R? Do you know what your output tube currents are at idle?
Just because you have slightly higher B+ doesn't mean you need to push you output power to max at all playing time.
Let us know where you are with quiescent conditions.
You can increase resistors for G2 supply and Preamp +rails, if you need to.
There's plenty of Fender Deluxe Rev with PS at 420V plus (fitted with6V6), Power Transformer 330-0-330VAC.
So at this stage with SS Rectifier and correct output tube current, I could see that HT come down. Note: nominal 330V x 1.41=467V DC  Allow a little extra for no load, that should be marginally higher, then when EL34's are fitted correctly, that I imagine will easily drop by 20 to 30V. My experiences, especially with Tx 660V CT and current rating of 140mA.
My thoughts and kind regards
Mirek

[Markbou]

Hi and thanks for being patient.

My meter is not sensitive enough to obtain a good voltage drop reading between the plate voltage and the OT center tap voltage, so I installed 1 ohm resistors between the tube's pins 8 and 1 that are tied together, to ground.

I cannot measure any millivolts across the 1 ohm, so if my meter is correct, no current is flowing through the cathode to ground.

Also, I swapped out the preamp tubes one at a time but the problem persisted.  I have a feeling the issue is right in the preamp because the hi input plays a little although squelchy, and lo doesn't play at all.  After a minute or so, the hi input stops playing too.  I inspected the circuit twice point by point and the connections all look correct.

As well, I'm seeing something I've never seen before.  Today, when I first powered it on, the voltage readings were off the chart, as in OL on the 600v setting, so I turned it off in fear, searched around a bit, and when I returned to it and powered it on, the voltage on B+1 was 398 volts.  Plus yesterday at one point the B+1 was measuring 474v.  I still have to set the bias but currently it is set to max cold; could that be a contributor.

I didn't expect to have to elicit the forum's help, but I had to seek out a source of knowledge.  And so thanks in advance for any comments.  I'm really looking forward to hearing how this amp sounds.

[sluckey]

What kind of output tubes? 6V6? EL34?

What voltage do you measure on pins 3, 4, 5, and 8 of the output tubes?

Which meter do you have?

[Markbou]

2 x EL34

Part of my confusion was the meter setting (brain fog.)  Below are the DC voltages on those pins.

Tubes 1 & 2
Pin 3 = 471v
Pin 4 = 469v
Pin 5 = -52v
Pin 8 = 0v

My meter is a cheapie, but it's worked well for a long time.  'Commercial Electric'

[Markbou]

What output transformer are you running?

I'm using a Hammond 1760H and connecting the 16 ohm jack to an 8 ohm speaker to accommodate the EL34s.

[AlNewman]

Doesn't sound like your output tubes are conducting.
Your meter should be good enough to measure dc voltage at the grid of your output tubes.
Maybe there's an issue with your bias supply.

Edit: Sorry, just saw your post. 

I dunno, just a quick look at the datasheet, looks like the grid should be about 30v?  That'd probably help with your plate voltages.

[Markbou]

Thanks for chiming in.  You put into words a vague thought I had based on the performance of the amp as is.  The guitar plays through the amp via the hi input when you hit a solid chord but then it fades quickly to nothing - like a conduction issue in the power tubes.  As well, if I have the guitar plugged in and turn the standby and power switch on at the same time, as the signal becomes increasingly audible - this ramp up in volume sounds like a completely healthy amp until it reaches it's crescendo then fades out until another chord is struck with decent force.  For the first 10 or 15 seconds of startup it sounds like it does if I do the same thing on one of my other amps.  These two tubes are a pair of new J-Js that came wrapped as an apex matched pair; my personal meter disagreed, but they are close enough for a guitar amp for sure.  The other thing I noticed is the guitar will play spottily through the amp for a couple of minutes then stop.  I have to shut the amp down for 10 minutes and power it back on to get it to make any sounds.  The DC voltages on the 12ax7 tubes are hovering around their limit of 330v DC.  At the very least, I know I will have to increase the droppers, but I don't think that is ultimately causing this.

[Markbou]

Doesn't sound like your output tubes are conducting.
Your meter should be good enough to measure dc voltage at the grid of your output tubes.
Maybe there's an issue with your bias supply.

Edit: Sorry, just saw your post. 

I dunno, just a quick look at the datasheet, looks like the grid should be about 30v?  That'd probably help with your plate voltages.

Going to go try that right now.  Thank you.

[Markbou]

Getting somewhere now.

Now, when the amp is on and a chord is struck it fades out a lot closer to normal.  It doesn't just cut off, it continues to fade back to the floor noise which feels like progress.

The voltages stayed the same on the 3, 4, 5, and 8.

I realized just now, I was supposed to use a 50k bias pot rather than the 25k one I used.  The result is I can only bring the bias voltage down to -44 volts, so tomorrow night I will switch it to 50k and report back.

Thank you all again.  I very much appreciate it.

[glass54]

Pin 5 = -52v

That seems to be a little high?? I would expect more like -39V to -43V Range at +430V to +450V HT. (For EL34)
Can you measure across 1R (Pin 8 to Ground) Don't need any special voltmeter, but preferable low DC Volts or Autorange (DC Volts).
Regards
Mirek

[Markbou]

Hi.

Yes.  One measured 50mA and the other 80.  I think besides the bias voltage being too negative, I believe I have to adjust the balance pot as well.

[Ronquest]

Posting your schematic can help others help you.  You have less than optimized parts for your objective.   

Marshall's 20 watt Origin runs cathode biased EL34s, low voltage ~270V, and a 5.4K output transformer.  Just one method. 

[PRR]

...My meter is a cheapie, but it's worked well for a long time.  'Commercial Electric' ...

Probably not relevant, and I know you'd have to go look, but..... it sometimes matters WHAT meter, Make and Model. 'Commercial Electric' is Home Depot's house-pet electrical brand, all of it made by many different makers. I see a dozen 'testers' (meters) in stock today and I'd bet they are brokered-in from many Asian makers.

If you may need help with "the meter setting" (we all do some days) then knowing a 426-RS from a IM-26 may be key to the de-fogging.

Unless you have an 8K OT, you will be above 25 Watts output with this PT. Since I assume you'd rather not start over with another smaller PT I guess you will have to put up with 30 or 35 Watts output.
> Marshall's 20 watt Origin runs cathode biased EL34s, low voltage ~270V, and a 5.4K output transformer.

Salvaging a burned-out VT40, I arrived at 23 W out with 6.6k load and 390V DC and 250r shared cathode resistor with 2k common screen resistor. (Rather shy of the original over-loud over-stressed 590V 60 Watt condition, but now likely to out-live me.)

[Markbou]

Thanks for that.  On Rob's website it says I can run the EL34 through the 16 ohm tap into an 8 ohm speaker, so that's what I was aiming for.

I intend to drop the voltage a bit once I get my bias under control.

I just finished swapping the bias trim pot but then I realized that even when I set the bias pot to zero, I can only get the bias voltage down to -43v, so the swap made no difference.  However, not sure where to go from here to get the bias voltage into the -30 to -40 range which is needed for my plate voltages.  A larger bias filter cap?  Thanks again for the input.

The circuit is this one from the robrobinette.com site, https://robrobinette.com/images/Guitar/Marshall/RR2104_Master_Volume_Micro/RR2104_Master_Volume_6V6_Schematic.png.

[sluckey]

I just finished swapping the bias trim pot but then I realized that even when I set the bias pot to zero, I can only get the bias voltage down to -43v, so the swap made no difference.  However, not sure where to go from here to get the bias voltage into the -30 to -40 range which is needed for my plate voltages.  A larger bias filter cap?  Thanks again for the input.

To decrease the bias voltage range, increase the size of that 470Ω/2W resistor. I suggest 1K, then recheck your bias voltage range on pin 5 of the output tubes. Experiment with different resistors until you are satisfied.

Remove the output tubes until you have the desired pin 5 voltage. They are not needed at this point and there's no need to stress them while experimenting on the bias circuit.

[Markbou]

Thank you so much.  I've tried a few values already and understand what I'm looking for.

Once I dial it in properly, I'll report back.

[Markbou]

@sluckey & everyone who commented to help me get this build squared away... THANK YOU!!

I currently have the amp running at 435v on the plates with 38.3 mA and 41.1mA flowing through the cathodes, with a bias voltage of -28.8  The preamp plate voltages are all hovering between 238 - 268v.  All that is left to do to this amp is attach my new Celestion 80 to it and tweak the sound using the bright caps and treble peakers.

To get to this point, I had to do the following per the suggestions in this thread due to using a 330-0-330 transformer instead of a 290-0-290:

1. Changed the bias resistor from a 2-watt 470 ohm to 2-watt 2k ohms.  This allowed a tube bias range of approximately -27 volts to -44.  Previously, with a 470 ohm resistor the range was approx. -38 to -52v

2. B+4 dropper increased from 10k to 18k

3. B+3 dropper increased from 10k to 27k

4. B+2 dropper increased from 6.8k to 12.4k (6.8k+5.6k)

The amp has a nice growl and sounds very close to a Marshall.  So far, I am impressed with this it and look forward to the speaker being broken in.  Someday in the near future, I plan to build another one of these for my 11 year old nephew when he's ready for it.

Thank you again for the help.

I wish everyone a great holiday season.

[JPK]

Glad you got it all sorted. So in hindsight what Hammond transformer would have been better? The 290AX is has both 325-0-325 and 275-0-275 but has no bias power taps. Is 290-0-290 what you were aiming for? I saw you post that voltage in an earlier post. 275-0-275 might be too low? Just wondering in case I wanted to build one of these. I just finished building a Ceriatone 2525 which is basically a Marshall mini Jubilee with no 5 watt option. Fantastic sounding amp but a pretty difficult build. I was very happy when it worked with no issues (used an old thread here for power up procedure, thanks especially to sluckey for the tip to check the bias voltage pot range before installing tubes). The JCM 800 6V6 might be a little too close in sound with the one I just built but I like building these things.

[Markbou]

In hindsight, I would have used a 290-0-290 and just tapped off the one of the high voltage legs for the bias.  The 330-0-330 I used had the 50v bias tap, so I went with that one.

I haven't checked the amp with a pair of 6V6 tubes yet, only the EL34s.  I do have an NOS pair of black plate 6V6 tubes from the 1950's I found in a partly built amp kit from that era, so I am looking forward to that too.
 I still need to add some treble to the sound but that is a simple process.

With this amp, I was looking for a stand-in for my 50w combo to make it easier to be mobile and this amp fits the bill perfectly.  I has a wonderful growl and the classic Marshall pre-amp sounds.  Love it!