Bonnet Box conversion 4x EL84s

[shaun]

Heyo. I have a dead Guild Bonnet Box record player - boy are they ugly. But it has a very cool amp in it to rebuild: EF86, 12ax7, 4x EL84 PTs. The iron looks great.

I want to re-purpose the cabinet and existing speaker(s) as a guitar amp, and I am hoping for suggestions as to which way to go amplifier-wise. I'd like to keep the EF86 as one input, and maybe add a 12ax7 input. I'd like to add a spring reverb. I suppose I could go the AC30 route, but there are many variations of that model that I'm not familiar with. I prefer simple, and I like to wire point-to-point - it helps me understand the schematic better, especially as I haven't been doing this for very long.

I've had success building two Hoffman designs (GA 5 Schedule 40, and a basic Marshall style 18watt PP), and I'd certainly be interested if there was a Hoffman version of an AC30 I could study. 

When I opened the little draw in the cabinet, it had the buyer's manual, and lo and behold, it had a schematic and voltage readings in it - yay!

All suggestions welcome, and thanks in advance.

A few minutes after posting this, I found the Hoffman AC30 shcematic, but I'm still open to suggestions. Thanks.

[shooter]

suggestions as to which way to go amplifier-wise.

I would get what you have working, I didn't study the schematic but quick look, you are 80 - 90% there guitar wise, once it's working and you have guitar speakers wired in, THEN you can tweak for tone, cap here, voltage divider there

[shaun]

Yeah, I was wondering about that course of action - it seems pretty intact. I'm very unfamiliar with the old EF86, but I suppose I can simply follow what they have there already.
Thanks Shooter.

[PRR]

Where is the Volume control??

Ah... there's another TEN tubes in a tuner/selector/preamp. Half those tubes could be audio stages. You could build several different channels and reverb also. Show us that chassis and plan.

Nermind. Here's that chock-ful-o-tubes chassis:

[shooter]

I think this link will get you where you want to be for a "close" to original preamp
Sluckeys AC15 lite
http://sluckeyamps.com/hammond/hammond.htm

the multi tap TS is probably "optional" 

[sluckey]

That schematic is a power amp. Build whatever guitar preamp you want and just plug into the input.

[shooter]

is a power amp

what did I miss, if he wires it up AC15 lite, same tubes through PI 

[sluckey]

You didn't miss anything. It could easily be an AC15 (ugh, AC30 lite). Tubes are right. Ditch the UL OT connections. Ditch the NFB back to the EF86.

But I would rather treat that entire schematic as a power amp because of the NFB wrap around. Just might come up with the next perfect tone. 

[shooter]

because of the NFB

see, I did miss something, thanks
 

[shaun]

Sluckey's AC15 lite project looks great, but I'm relatively new to amp building and don't know how to extrapolate a 2x EL84 into a 4x EL84 without a schematic.

After considering the options, here's what I'm thinking:

1: No reverb or tremolo because this project will be complicated enough for me without them.

2: At 120vac in, the Bonnetbox PT puts out 352vac per plate, which seems high. They have 22ohm resistors on each plate: is that to drop the vac going into the plates? The resistors didn't seem to change my meter reading at all, but maybe I'm doing something wrong.

3: My limited experience with untried PTs is that, when the plate vac is high, the B+ coming off the rectifier cathode will also be higher than desired for a conversion to a guitar amp - in my builds, it seems my B+ is always at least 30v higher than hoped for, which makes for some interesting experiments:). But I'd like to nip that problem in the bud with this build, and I believe that to do so, I'll need a design with a filtering system that can tone down that voltage. The Hoffman AC30 is a very nice schematic, and I'm a big fan of other Hoffman's designs I've built (2 so far), but the filters in the Hoffman AC30 are 16uF and 8uF, so I'm assuming that his transformer would put out much less vac and is purposed for that particular design. The Bonnetbox schematic has 40uF filters. Can I simply increase the filters in the Hoffman AC30 design to tone down the B+ that will come off my rogue PT? I'm sure there must be a good tutorial on the effects of filters on B+ generally, but I haven't found one yet. I noticed Sluckey's AC15 has a large resistor bolted to the chassis and going to the rectifier. It seems that's one way to control the voltage - is that correct?

4: I'd like to experiment with a two channel input, one with 12ax7 and one with an EF86, but again, the Hoffman schematic doesn't use the EF86. I can probably find a Vox preamp design and copy their use of the EF86 though, so no worries there (hopefully).

Thanks again for the help I've received on this forum. It's very cool that you guys are so giving of your time and expertise. Cheers.

[PRR]

> 352vac per plate, which seems high. They have 22ohm resistors on each plate: is that to drop the vac going into the plates? The resistors didn't seem to change my meter reading at all

It worked in 1959. And probably for decades more until the e-caps quit.

Yes, line voltage rose and you won't/don't have the heavy tuner load (this is accounted in the voltage chart; it was surely intended to play records all day long). But I sure would not worry in advance. This is not a moon rocket.

> my B+ is always at least 30v higher than hoped for

10%? Bah, not important.

And once working, you can trim B+ by changing the 75r 10W in the first B+ node.

But the historical trend is for guitar amps to be run HOTTER than living-room amps. More power per pound to lug, and a working musician can afford the shorter tube life. (Typically less than drycleaning your good shirt.)

Yes, leave the stock power supply ALONE. (Except replace perished parts.) It is a fine design. Perhaps 50 cents more cost than a stage amp "needs", so what?

> two channel input, one with 12ax7 and one with an EF86

The Tuner unit *has* a pentode preamp (for phono), with Bass/Treb Bax tone control, and a couple 12A_7 sockets which could be rewired for 12AX7. I realize the tuner is a large lump. If you have the whole Early American cabinet, it would be a stunning living-room amp (add iPod input). The stock speakers would be ample in living-room. If you take it to clubs and fight a drummer these speakers won't last, but g-speakers would fit.

[Tony Bones]

My advice: don't worry too much about where you want to end up, but instead look forward to an adventure along the route, wherever it goes.

Start the journey by replacing the input jack with a 1/4" phone jack. If it works, then plug in a guitar and wail away. Use the volume knob on the guitar. Next try disconnecting the global negative feedback (from the speaker terminals back to the EF86.)

After that....?

There's a good chance that you'll need to replace all of the electrolytics, as well as a few coupling caps to get it working. There's a huge risk that you'll learn more than you expected.

It's not as though you need the amp right away, right? So take it one step at a time. Decide which way to turn when you get to each intersection. Put it on the shelf if you get bored. Put it back on the bench when you feel motivated.

[shooter]

extrapolate a 2x EL84 into a 4x EL84 without a schematic.

your initial schematic "IS" the INTO.  you already have a 4X fix whats broke, then jam, then tweak, then jam.......

[shaun]

Hi All. I've been hunkered down working on this project non-stop for what seems like a year, although is only a few weeks. And once again, I've hit the dreaded wall of ignorance. When attempting to convert this unit to guitar amp, I first want to understand the original system and keep it as original as possible. But things seem to go sideways, somehow.

I replaced all electrolytics but am now getting excessive B+ voltages:
GZ34 pin 6 - 433v;   EL84s pin 7 - 413v;    12ax7 PI pin 1 - 361. The B+ on the EF86 has remained a rather tame 76v, which is what the manufacturers stated it should be. (There's a voltage chart posted at the start of this thread, along with the schematic.)

As part of my reconfiguration, I repurposed the octo plug that used to connect to the tuner/turntable. Because I no longer needed B+ going to an external chassis, I removed the 3k 20w resistor that ran from the second filter cap (40uF) to the octo plug. This may have been a mistake because, when I reconnected that resistor and ran it to ground, it tamed the excessive voltages downward considerably. The only problem is that the 3k 20w resistor gets extremely hot - hotter than the EL84s (measured scientifically by holding my hand as close as possible while trying to avoid being burned:).

W/out 3k resistor: Rectifier pin 6 - 433v;    EL84s pin 7 - 413v;    12ax7 pin 1 - 361v.
With 3k resistor:   Rectifier pin 6 - 380v;     EL84s pin 7 - 360v;   12ax7 pin 1 - 290v.

As a point of interest, the original PT rectifier was designed to supply another eleven - count 'em - tubes in the tuner/turntable chassis. So it probably supplies way more power than I need.

I'm wondering about my options: Should I keep the 3k resistor? If so, it gets too hot to run as is, so it would need to be modified somehow. Second, the new cap cans I installed each have an extra 20uF cap in them; can I used those to create another filter/resistor combination that would help tame the B+? Finally, should I simply switch out the GZ34 to something with less output? Or all the above?

Thanks for any help you can give that will allow me to scale the dreaded wall of ignorance. I suspect that, on the other side of that wall, I may encounter the even more dreaded wall of stupidity, but time will tell.

[bmccowan]

There is no wall of stupidity (yet) only a lake of stupidity, and we all go for a swim once in a while.

[shooter]

I removed the 3k 20w

schematic shows 33K which will still get HOT
What voltages are you aiming for, you can tweak the "rail" Rs to drop Volts, the 75, 6.8k.....

EDIT:
looking at wrong R 

when I reconnected that resistor and ran it to ground

did you look n see if it went to ground on the other end 

[shaun]

Yes, it did, as part of a power filter section that was within the second chassis.

But I understand the 33k mix-up; it's a 1/2w, but it runs to ground also. So,following that example, I assumed I could run the 3k 20w to ground also. But man, it gets hot enough to fry eggs - smoking even, although I think it's simply burning off old flux nearby.

[sluckey]

I assumed I could run the 3k 20w to ground also. But man, it gets hot enough to fry eggs - smoking even, although I think it's simply burning off old flux nearby.

Don't do that! That 3K/20W was never meant to connect to ground. Look at the schematic PRR posted.

[PRR]

Change GZ34 rectifier to a higher-loss 5U4.

Change "75r 10W" to perhaps 500r 20W (two 1k 10W) (still needs big space for cooling).

[shaun]

Thanks, PRR. I'll do that and post the results. There is room around the 75r 10w for cooling, so I'm keen to give it a try.

Very grateful for the great support and info on this forum - guys like me are lucky to have you guys for support and advice. Much appreciated.

[tubenit]

Change GZ34 rectifier to a higher-loss 5U4.

Regarding PRR's excellent advice, this "tube cheat sheet" may help make sense of this IF you have not already figured it out. I am sharing this since you've conveyed that you're somewhat new to tube amp building and still wanting to learn.

The GZ34 is "sort of" guesstimated 1.3 x VAC = ________.    So, if you're using a GZ34 and have 390 volts on the plates of your power tubes, then your PT may be a 300-0-300.    (300 X 1.3 = 390)

The 5U4GT is 1.2 x VAC.  So, with the same 300-0-300 PT then you'd get 360v on the plates.    (300 X 1.2 = 360)

You have four EL84's.  Each pair draws around 90ma of current.  So the quad of EL84's draw about 180ma.  So, you'd need a rectifier that could supply/handle that much current.  The one PRR suggested will do that.

With respect, Tubenit

 

[tubenit]

So, let's say it's accurate that you have 380v on the plates of the EL84's with the GZ34.

We would divide 380 by 1.3 and have a "guesstimated" 292-0-292 PT (maybe).

So using a 5U4, we would guesstimate 292 x 1.2 = 350v on the plates of the EL84's.  The max voltage rating of EL84's is around 350v.  So, with good tubes, you might be OK with that?

With respect, Tubenit.

[tubenit]

Let's say that you felt like you wanted to be further below the 350v on the plates.  Well, you could consider using a 5Y3GT.

5Y3GT is 1.1 X VAC guesstimated.  So  292 x 1.1 = 321v.   However, there is a problem in that the 5Y3GT can NOT supply enough current for a quad of EL84's.  So, what you'd have to do is simply only use a pair because a pair of EL84's is gonna draw a guesstimate 90ma and the 5Y3GT can supply 125ma.

Now, my personal thinking on this is that 36 watts is ALOT of volume!  I would NOT want that much volume because I would never use it especially at home.  So, IF it were my amp ………………  I'd pull two EL84's & use a 5Y3GT and run 18watts which should still be ear splitting loud.  Just a thought to consider, not saying you should do this.

With respect, Tubenit

[PRR]

> your PT may be a 300-0-300

The image Ugly Betty voltages1.jpg in the first post tells us 325VAC, which is *with* the ~~60mA DC of the tuner.

Working at 3/4 the current, DC voltage will be high.

I'm getting estimates like 320VDC, which does not seem excessive for EL84.

[tubenit]

In reply #13 by the original poster, I thought he was saying the rectifier was producing 380v?

With 3k resistor:   Rectifier pin 6 - 380v;     EL84s pin 7 - 360v;   12ax7 pin 1 - 290v.

Given his reported actual 380v on the rectifier,  I gave the guesstimated/maybe idea that the PT was 292-0-292. I have no way of knowing whether the PT is original …… or if the reported voltage chart was truly accurate  (or perhaps like many Gibson schematics, not representing the actual amp).

So, let's say it's accurate that you have 380v on the plates of the EL84's with the GZ34.
We would divide 380 by 1.3 and have a "guesstimated" 292-0-292 PT (maybe).

The comment in reply #20 about "a" PT being 300-0-300 was simply in relationship to an illustration explaining how rectifiers work.

And IF he is getting 320V on his EL84 plates with a 5U4, I completely agree that is not excessive at all and a good working voltage.

With respect, Tubenit

[shaun]

Hi all,
I took measurements that may explain my conundrum. First, just to clarify once again, here are the actual original readings with 120VAC going into the amp from the variac:

GZ34 pin 6 - 433v;   EL84s pin 7 - 413v;    12ax7 PI pin 1 - 361v. The B+ on the EF86 has remained a rather tame 76v, so at least that is sitting pretty.

In attempting to lower the B+, I tried installing a GU4 as suggested, which trimmed about 10 volts off the B+. I also installed a 3k resistor in place of the 75 ohm 10w. This is is a considerable increase, yet it trimmed only another five volts off the B+. So the B+ is still way high. I don't feel comfortable running EL84s at 400v, nor the 12ax7 at 330v, so I have to tame the transformer way down. I simply don't know how, other than to replace the transformer.

An initial question concerned whether adding an extra filter stage to the amp would help soak up some of the voltage - I could easily do this because the new cap cans I installed each have a 20uF section extra that are presently unused. After all, the tuner unit that was part of the Bonnet Box system had a 40uF + 20uF cap can in it, which would have added considerable power conditioning to the B+ going into the tuner amp, and thus also probably affected the B+ in the power amp.

Maybe I'll add another filter stage and see what happens, as increasing the resistors has had limited results.

Thanks again for your help.

[shooter]

what voltage do you get at Pin3 of the 84's

your biggest load will be at the 1st tap, so adding anything "downstream" won't do much unless you hang more 84's down there

[shaun]

Pin 3 of the EL84s = 6.6vdc

And that's with the 5U4 reducing the voltage down a little, compared to the stock 5AR4.

[tubenit]

Shaun,

Please be forebearing with me ……………    Is there any chance your voltmeter needs a new battery?   The reason I ask this is because on a build yrs and yrs ago, I kept getting high readings that were driving me crazy.  Turned out the voltmeter needed a new battery and when I replaced it, all was well with voltages?  It wouldn't hurt to rule that out if you have not done so already. 

There "should" be more of a drop in the B+ voltage then 10 volts with the 5U4 instead of the GZ34.  Something seems amiss? 

Let's say that's not the issue?  What does the GZ34 produce with NO other tubes in the amp?  Is the voltage remaining the same or the B+ voltage increases with NO tubes?

Let's say at some point you conclude that your PT is simply not a good match for the EL84's?   You could certainly eliminate the 4 EL84's and instead use JJ 6V6's at 400 + volts.  I ran a guitar amp with 476 volts on the plates of JJ 6V6 for 7+ yrs and never had an issue with them. The amp eventually sold with those same tubes.   Meaning you could build Sluckey's AC15 topology but with 6V6's or some other amp.

Just some thoughts to consider?  I'm puzzled on this one unless it proves to be the voltmeter doesn't work well?

With respect, Tubenit

[shaun]

Good question, tubenit! I'll check it and see.
I'll take further measurements and post them in a few minutes.

[shaun]

The meter's battery tests good.

The B+ with only the rectifier in is 480vdc at node 1.

It's interesting to learn that 6V6s can run that hot. Thanks for the info!

[shooter]

6.6vdc

pin 7 - 413v;

(6.6/130)/2 ~ .025A
.025*413 ~10.3W
what I don't know is if B+ outweighs max dis

if you're in a playful mood, change the 130ohm to 220ohm
redo math, play

[PRR]

> installed a 3k resistor in place of the 75 ohm ....it trimmed only another five volts off the B+

5V in 3k is less than 2mA. This suggests the EL84s are not working at ALL.

With all four EL84s working we expect 150mA at 320V. With 3k flowing 150mA we calculate "450V drop". Which is impossible (you'd really be near 140V 70mA).

So something ain't right.

[shaun]

Hmmm. Very interesting, PRR. I trade out the EL84s and see if it make a difference. Then I'll experiment with the 130r.

Thank you!

[tubenit]

Shaun,

Can you confirm what the PT voltage is with OUT any tubes in the amp?  I am truly puzzled by the high voltages?   And do you think the original PT was changed and this is a replacement of some sort?

With respect, Tubenit

[shaun]

Yes, it's 348 across the CT with 120v on the variac going in, so I guess that would make it 696 across the main taps. Which seems good consider no load and 120v instead of 115v.

So PRR's suggestion seems a good one. I've ordered a new set of EL84 quads. I'll retest and post my findings.
Thank you!

[tubenit]

I hope I am mistaken and I might be …………….   

My thinking is that you will not be able to achieve the voltage that you want on the EL84's with that PT?   Example:  the Classictone PT 40-18050 for a Vox AC30 has  280-0-280 and provides 270ma.

280 X 1.3 (GZ34) = 364 volts     (with 350v typically thought of as max volts for EL84's)  AC30 are reported to be rough on EL84 power tubes.

You report your PT is 348-0-348  (if I am understanding you correctly).   348 X 1.3 (GZ34) = 452 volts !

348 x 1.2 (5U4) = 417 volts.

Let's assume with the tubes loaded in the amp, it pulls the voltage down some.  I still would be surprised if you can the plate voltages low enough for the EL84's?

From reply #25,  you report :   

GZ34 pin 6 - 433v;   EL84s pin 7 - 413v

   AND you state that using a 5U4 only lowered the B+ voltage by 10 volts.

IF you have the "wrong" PT for the EL84 tubes, you could use a pair of JJ6V6's and probably make the PT work OK? 

I will be surprised if you are able to make the current PT work for you using a quad of EL84's?  (I think changing to new EL84's may not make a difference in getting you there, but I could be wrong?)

With respect, Tubenit

[shaun]

Thank you for your interest, tubenit. I agree, it seems odd, but the PT came as part of the original amp in the Bonnet Box - at least, everything looks original, and there is no reason to suspect anything has been changed.

The 5AR4 tube is okay, so changing the power tubes seems like a good next step. I'm waiting on their delivery.

[tubenit]

I genuinely hope it works out for you.  So I'm rooting for you.  Maybe something about the new tubes will make a difference OR maybe you'll find out what's elevating the voltages.  I had really hoped you simply had faulty readings on your voltmeter but you've ruled that out.

I will share with you that I've had EL84's redplate on my and start oscillating at barely over 350v and ruin a tube before. So, IF you are still getting beyond 360v on the EL84's, I hope you'll consider shutting the amp down until you can figure out how to get a voltage closer to 350v.    OR ……….. resolve to switch to JJ 6V6's that can handle that type of voltage.

Respectfully,  Tubenit

[shooter]

I'm rooting for you also
I'm still somewhat confused since (if I did the numbers correctly) I got 10+w a tube
doesn't mean their good, but seems they/something is drawing current which shoulda reflected in the PS R changes 
but 413vdc is not a good value for B+ on '84s

[sluckey]

Shaun, I just read through all of your posts in this thread. I don't see any mention of the EL84s ever being plugged in while you measured all these high voltage readings. In fact, this statement, "I also installed a 3k resistor in place of the 75 ohm 10w. This is is a considerable increase, yet it trimmed only another five volts off the B+. ", seems to support that notion. So, were the EL84s plugged in when you made these voltage measurements?

Also, there's no mention of what the amp sounds like as it is. Not even to say it just makes no sound, or a loud hum, or squeals like a stuck pig, etc... What about that?

[shaun]

Thanks for the great questions! I measured with tubes in place, limiting the amount of time I subjected the tubes to the high voltages. Then I experimented and tested voltages with only 65v reading on the variac to see if anything I did created a change for the voltages generally, but nothing has turned up that would indicate why the readings remain high. There is very little noise coming from the amp other than a low amount of hum - too early for me to tell if that's a problem or normal. It seems pretty quiet, which is good, but there is hardly any guitar signal getting through to the speakers either, so that's not so good and could be an indication of bad tubes somewhere, although I believe the preamp tubes (EF86 and 12ax7) are good.

PRR suggested changing the EL84s, which lead me to consider the possibility that a bad power tube could be shorting something, resulting in high voltage - I'm still too much of a novice to know everything I need to know, so diagnosing such problems takes me a long time. In any case, a new set of power tubes will remove one unknown, and they'll be here in a day or so. In the past, I've been surprised how changing tubes can make a big difference, so I have my fingers crossed.  If that's not the problem, I'll move on to the next likely suspect, whatever that becomes.

I suppose you gotta love a mystery to be into amp building.

[sluckey]

Well, those EL84s are not drawing any current. They are either all dead or something else is preventing them from drawing current. Check the voltage on pin 3 of all four EL84s. What have you?

Until you get those EL84s conducting normally, thus putting a load on the power supply, your voltages will be high. This is normal. I would not fret over it.

EDIT... Oops! Just noticed you said pin 3 was 6.6V. That translates to about 25mA per tube. Not as much as expected, but at least the tubes ain't dead.

[PRR]

... there is no reason to suspect anything has been changed.

Except (I assume) you disconnected the tuner. That's a ~~60mA load, lost. The drop of total (estimated) load from 220mA to 160mA is sure to un-sag the factory voltage.

I don't think 60mA of dummy-load is wise. (24 Watts waste heat!)

Increasing that power resistor I cited will directly increase drop without huge extra heat.

However if it really is sucking 25mA per EL84, 100mA total, something IS wrong. Have you compiled a list of voltage at each pin of each EL84? Have you verified the values of those cathode resistors?

[shaun]

Boy, it's great when something makes sense! I installed new EL84s as per PRRs suggestion, and voila - Rectifier B+ = 349v;  EL84s plates = 304v;  12ax7 PI plate = 279v; EF86 plate now =a measly 40v.

So that's the great news. But of course, there's always another challenge. This time, it's probably more fixable. There are two issues, one of which is possibly due to my wiring somewhere in the tone stack or preamp; there is a good signal from input to preamp, and only a faint hum at full volume, but when the volume is turned down, the hum goes considerably louder. Second, there is a gentle low-toned motorboating occurring. I could be off with my thinking on this, but I wondered if it had anything to do with the fact that there was a two-section filter cap in the tuner that was removed - 20uF and 40uF. The new cap cans I installed in the main amp each have a spare 20uF section, so there may be potential to add them to the filter section somehow, but I'm not sure exactly where to insert them, if at all - preamp filtering? Power filtering?

In any case, it feels great to be making progress. Thank you all for the great help.

[tubenit]

Congrats on getting the B+ voltage at a good place for the EL84's!!!  Glad that worked out for you.  PRR & Sluckey called that one correctly.   

Any chance you can post a schematic of what you actually have at this point?  Even if it's a cut & paste schematic?

With respect, Tubenit

[shaun]

Thanks, tubenit. My schematic drawing skills are not up to much yet, unfortunately (any recommendations for an easy-to-use software would be appreciated).

However, you have the original amp schematic already, so all I did was add a control panel with a vol and tone pot. I can explain how I did that, and I would be happy to post pictures if you'd like to see my progress (just let me know - I don't see too many pics on these threads).

My current big question concerns grounding. I've connected a ground lead from the power amp chassis to the control panel (control panel is made of metal). The control panel has an on/off switch and a 6.3v lamp (tapped of the PT, and with a 100ohm res to ground on both legs). I think I have that figured out okay.

The problem may lie in the signal lines. I used shielded cable going from the amp to the control panel and back (send and return),  with the shielding attached at one end to RCA plugs going into the amp. However, at the control panel end of the signal cables, I also attached the shielding braid to the tone and vol pots , so I may have created a ground loop. I've heard shielding should only be grounded at one end, but I didn't know if that applied here. I'm just guessing, sorry to say. But it may explain why the volume works when turned up full, but when turned down, it creates a big hum.

Cheers all. Hope you're having a great Sunday.

[tubenit]

There is an easy to use FREE software for drawing schematics called ExpressSCH.  AND …… we have a library of 100's of schematics and layouts using this program.  The drawings are easily editable.  It is a very user friendly intuitive program, IMO.
http://el34world.com/Forum/index.php?board=12.0

I would ground just one end of a shielded wire to avoid a ground loop and adding unwanted hum.

I personally have never done an amp connecting a separate preamp chassis to a power amp chassis. So maybe one of the guys who has done that can offer you some tips.

With respect, Tubenit

[shaun]

Easy question!

I know this thread has dragged on a bit due to my inexperience- my apologies. But I am making progress, so thank you for the help.

This should be an easy question .
Referring to the schematic: coming off the plate of the EF86, as we follow the signal path, there is an unfamiliar (to me) little by-pass system - at least, that's how it seems. There is a .002 cap running to a 100k resistor, both ends connecting to the B+. I'd like to understand why that is there, and why there is no decoupling capacitor between the EF86 and 12AX7. I want to insert a modest tone stack after the EF86, and I need to understand this system before doing so.

Regarding the amp generally, I've used a 5U4 in place of the 5AR4, which has dropped the voltage by 35-40 volts on the PI and EL84s. They're still a  little high, but should be okay: B+ is down from 409v to 371v; EL84s now 345v;  12AX7 now 314v.  I'll experiment with increased plate resistance on the 12AX7 to bring it down a little further, and maybe on the EL84s as well, but I think they can run hot, so I want to get the preamp end sorted out first.

Thanks.

[shooter]

This should be an easy question

I'll wait for the easy answer because I'm just gonna stab myself in the dark 

my stab, the cap/100k is your "tone-stack" 
the direct coupled stage scares me