Nasty sound issues in 5E8A (was: Rectifier Pin 8 Reads 0)

[BobL]

I'm having two issues... the first is finding the proper method for testing this.

I'm testing a new 5E8A build.

I am turning the AC on, standby off, with no tubes in the amp, and testing heater voltages as my first step - this is right?

I get 5.46v on pin 2 of the rectifier sockets.

I get 0v (or .036) on pin 8.

I get 361v on pins 4/6.

I get 3.51 - 3.53v on all power and preamp tube heaters.

I have tried reversing which yellow PT wire goes to which pin to eliminate that as an issue, and it seems to be something downstream, as the problem persists either way

I saw one old thread where a similar discussion took place, and people said that if the 'B+ rail' were shorting, then it would connect pin 8 to ground, and read zero... which makes sense, but I'm not sure where all to look for issues with the 'B+ rail', because I only have a vague idea of what that means.

I have inspected all tube sockets for shorted connections.

Where should I be looking?  

[John]

Hi Bob, very first thing you should do is build a light bulb limiter (image attached) That will allow you to see if you actually have a short or not,and also let you put in the rect. tube and measure voltages. I use a 60 W incandescent in  mine, some guys start with a 25 and work up. It was the first thing I built, and I use it faithfully.

The B+ rail is the main power supply line, it consists of all those "nodes" people talk about feeding the plates of the tubes. It's the grouping in the lower left corner of the Blackstar schem I've attached. Not all schematics draw it the same way, but most of us here do it similarly when drawing it out ourselves.

This doesn't answer your question about the 0 on pin 8, but someone else will be able to tell you exactly where to look, I bet. I think I'm missing something obvious about exactly how the rect. tube works, i.e. it has to be in the socket in order to measure correct voltages, but don't bet your PT on it. :) Build that limiter first.

Hope this helps!

*edit* I think the key is that pin 8 provides the ground to heat the cathode, and so with no tube in the voltage will be 0. Most rect. tubes use a directly heated cathode, which is different than what your other tubes do. The 3rd schem of the LC Studio shows the "normal" rect tube.  Hopefully I am not confusing you!

[Geezer]

+1 on the current limiter

Place your meter leads on pins 2 & 8.....you should read your ~5.46v

Without a rectifier in the socket, all you have (or should have) is the HT winding connected to pins 4 & 6 (reading the 361v from each pin to ground, since you have a center tap connected to ground), and the recto heater supply connected to pins 2 & 8 (which you must read from pin to pin, since there isn't a center tap on that winding..... No ref to ground)

Once the recto tube is in the socket, then things start to happen....the HT AC voltage is rectified into DC, which you will then be able to read on pin 8

[HotBluePlates]

I get 5.46v on pin 2 of the rectifier sockets.

I get 0v (or .036) on pin 8.

I get 361v on pins 4/6.

I gather that measuring pins 2, 4 and 6, you're checking a.c. volts. They all look good.

When measuring pin 8, were you set for d.c. volts? Restating something Geezer said, but in a different way, until you put in the rectifier tube there is no d.c. at pin 8 to measure. Put in the rectifier and try again.

If you want to be certain you don't mess up anything, unsolder the choke wire from the first 16uF cap in the power supply. Now, you only have the rectifier and that cap, but you will be able to see d.c. output from the rectifier. You can add other powe supply components to the chain once you're sure there's no issue with them.

[BobL]

Oh, so this actually would be normal if the tube wasn't in there... I wondered about that. I've tried to be very meticulous about going over the connections at each stage of the build - triple checking vs. the schematic/layout, and I've been over the tube connections with a fine tooth comb looking for potential shorts.

I was checking AC volts for all of these.

Thanks for the tip on the limiter - I'll have to make one of those!

[phsyconoodler]

Nobody mentioned if the tube was any good.......just sayin'....

[sluckey]

Nobody mentioned if the tube was any good.......just sayin'....

Well yeah, but he did say "with no tubes in the amp".   

[eleventeen]

"Oh, so this actually would be normal if the tube wasn't in there.."

Right!

[BobL]

Learnin'. :)

Thanks, everyone.

[Willabe]

I'm not sure where all to look for issues with the 'B+ rail', because I only have a vague idea of what that means.

B+dcv rail is in red box and the (negative) -dcv bias is in the blue box.


              Brad      

[Willabe]

For more clarity, I added the B+dcv nodes in yellow and the purple line is the B+dcv rail.

Notice each node has it's own filter cap(s) with a dropping R in between each node/filter cap. Most larger guitar amps have the choke in between the output tube plate supply and screen supply or just a dropping R in smaller amps. Your amp has a pi-filter (cap-choke-cap) feeding the plate supply.  

Blue line is -dcv for grid bias.

        Brad        

[BobL]

So it seemed to me like things were pretty much in the ballpark, so I fired it up, and to make a long story short, it does not sound good at all...

I have two main issues:

1.This is hard to describe, but single notes sound more or less ok, but start to dig in with repeated notes and it's like they die very quickly, and there is some fartiness to the sound, and the overall volume doesn't seem to be nearly as loud as I was led to believe this amp would be (I've heard that by the time you hit  6 on the volume, it's killing loud).

2. The presence control is useless, but in the opposite direction of what I thought it would be - which is to say, if I turn it all the way *down*, I get nasty, nasty distortion - sounds like static or digital distortion. 

I remember having similar issues with a JTM45 when I would turn the presence all the way *up* (squealy, crackly distortion).  With the JTM, I never ran the presence past 1, so it was never an issue.  This issue seems weird to me in that it is reversed.

Couple things: I had routed the presence to the back of the chassis because I was going to use a master volume on the front, so when things sounded wrong, I removed the master and set it all stock, and routed the presence back to the front of the amp - and the problem seems to be exactly the same.  I do have the lead to the presence control going above the board (which is to say, it goes to a hole in the back of the board, then back over the top.  I've tried to keep this lead away from other stuff, and moving it around doesn't seem to matter.

I don't have the chassis in the cab (I don't actually have a cab for this yet), so there isn't shielding over the back, but this alone doesn't seem like enough to cause any of this, at least not to this extent.  Something is clearly wrong with the sound... just not sure what.

I also put a pair of Sozo .1uf caps into the PI couplers just because I had them sitting around, used from the JTM.  I have both Orange Drops (716) and Mallory 150s I could replace in there.

So... tube tests... there are a couple that seem odd to me, but maybe are not - e.g. 0s on pins 2 and 7 of V1 and V2.

Maybe someone sees something below that is a clue?  Thanks in advance... sorry for the longwinded post.

Rectifier 1 and 2 measure the same:
With no tube installed:
Pin 2: 5.44 AC
4: 359 AC
6: 359 AC
8: .033 AC

With tube installed (is this weird?)
2 AC: Meter cycles through OL, 54v, and .7ish AC volts
2 DC: 445
8 DC: 445
4 AC: 350
6 AC: 350

Power tubes, with tube installed, amp fully on:
V6 (6L6WGC-STR):
1: 25.4 DC
2: 3.37 AC
3: 438 DC
4: 438 DC
5: -48.6 DC
6: -48.6 DC
7: 3.37 AC
8: 25.4 DC

V5: (6L6WGC-STR):
1: 24.7 DC
2: 3.36 AC
3: 437 DC
4: 439 DC
5: -48.6 DC
6: -48.6 DC
7: 3.36 AC
8: 24.7 DC

V4 (12AX7)
1: 220
2: 1.2
3: 1.765
4: 3.35 AC
5: 3.35 AC
6: 273
7: 24.3
8: 66.6
9: 3.35 AC

V3 (12AY7)
1: 128.4
2: 0.4 ??
3: 2.05
4: 3.35 AC
5: 3.35 AC
6: 262
7: 128.4
8: 131.2
9: 3.35 AC

V2 (12AY7)
1: 143.3
2: 0 ??
3: 2.37
4: 3.35 AC
5: 3.35 AC
6: 146.5
7: 0 ??
8: 2.37
9: 3.35 AC

V1 (12AY7)
1: 143.1
2: 0
3: 2.37
4: 3.35 AC
5: 3.35 AC
6: 144
7: 0
8: 2.37
9: 3.35 AC

[HotBluePlates]

So it seemed to me like things were pretty much in the ballpark, so I fired it up, and to make a long story short, it does not sound good at all...

I have two main issues: ...

Before trying to do any deep troubleshooting, assume you have wired something wrong (otherwise, the amp would be working properly; there hasn't been a chance for a part to fail).

You mentioned the presence control, so triple-check the wiring of the control and everything connected to it.

Get the presence working first. After, you may find a different problem, it it may all be fixed. If not fixed, assume another wiring error or wrong-value component. Triple-check again against the schematic/layout.

[sluckey]

Pin 8 of the power tubes should be connected to ground and should read zero volts, not 25v. Should be easy to see why that voltage is wrong.

It's possible that you may need to reverse the OT primary plate leads.

[BobL]

Power tubes: Pin 1 is tied to pin 8, and that ties to ground with a 1ohm resistor.

The plate leads are the brown and blue wires from the OT that we discussed yesterday?  I kept these long in case they needed to be reversed, but I guess I was expecting something more impressive soundwise if this was the case... though maybe this weird presence thing is manifestation of that...  I can certainly try that and see if it helps... though I guess maybe not while the wife and kids are sleeping 8ft above the amp. :)


HotBluePlates: I have definitely been over the wiring multiple times, and will continue to do so, but so far I haven't found anything obvious.

One question: I saw multiple places where people said that you shouldn't trust the chassis ground on the inputs, and to run the ground of the inputs to ground with a wire, even though this creates a small ground loop, because the loop is small and far from high voltage, so it shouldn't be an issue... so I did that, and the wire that does this (to a lug on the chassis) runs about 2 inches... that couldn't cause anything like this, could it?  My impression was that ground loops just hum...

[sluckey]

Power tubes: Pin 1 is tied to pin 8, and that ties to ground with a 1ohm resistor.

Well, you either read the voltage wrong or those 1Ω resistors ain't really 1Ω. There's no way to have 25v on pin 8 if those resistors are 1Ω. That would mean there is 25 amps flowing thru the tubes.

Maybe the measured voltage is really 25 millivolts?

[BobL]

You are absolutely correct.  25 mV.  My bad...

[HotBluePlates]

The plate leads are the brown and blue wires from the OT that we discussed yesterday?  I kept these long in case they needed to be reversed, but I guess I was expecting something more impressive soundwise if this was the case...

Yep, those wires.

Or... you could unsolder the wire from the speaker jack that runs back to the circuit board (which is the feedback signal from the OT), and see if the problem stops. If yes, it confirms the feedback/presence circuit is the problem.

The cure may be swapping OT plate wires, or it may be a different wiring error in the presence circuit. You won't know until you look.

HotBluePlates: I have definitely been over the wiring multiple times, and will continue to do so, but so far I haven't found anything obvious.

I've built a number of amps. Right now, I can't think of a single one that didn't have at least 1 minor wiring error somewhere. And after 20 years of dabbling in this stuff, my confidence that I wired everything exactly right is tempered with the knowledge that if I'd wired the amp perfectly then it would be working perfectly. The long experience just means I find and fix my mistakes quicker.

[ajeffcote]

I don't have anything to offer, I'm still green at this. But I'm curious if you used a ground buss across the back of the pots, or a plate or just the chassis ground? I did a 5E8A a little while ago, but I'm not real happy with the way it sounds so I'm following your thread.

[BobL]

I did a buss wire across the back of the pots, run to a lug under the board near the presence (was the shortest run to ground I could make).

[BobL]

I've built a number of amps. Right now, I can't think of a single one that didn't have at least 1 minor wiring error somewhere. And after 20 years of dabbling in this stuff, my confidence that I wired everything exactly right is tempered with the knowledge that if I'd wired the amp perfectly then it would be working perfectly. The long experience just means I find and fix my mistakes quicker.

Yeah, I'm not going to be surprised if/when I find a place I did something wrong... I'm just not finding it yet.

Just got home, going to try swapping those wires, then if still an issue, I'll try your presence test... and so on...

I sure appreciate all the advice!

[BobL]

Well, swapping those plate wires seems to have been the fix.

I think the 12AY7s will have to become 12AX7s...

Amp is still not as loud as I expected, though pretty loud.

I think I might be at the 'tweaking for taste' stage.

[HotBluePlates]

Awesome!

You have a 50/50 chance of getting those plate wires wrong. And in the wrong 50%, about half the time the amp squeals/howls at full powe as soon as you switch it on. That leaves about 25% of all your amps where you might have those plate wires wrong, and it doesn't howl but just doesn't sound right.

So a first guess is to swap plate wires and see what happens. "60% of the time, it works every time!"  "80% of statistics can be made to say whatever you want them to say... 60% of the time."

[BobL]

Yeah, I just moved my presence to the back of the chassis again, and it sounds totally fine back there.  Hard to get to... but... tomorrow, I'll try out my master again and see if that murders the tone or not.

[BobL]

I clearly have another issue... my voltages all look ok above, yeah?

I was so happy that the sound wasn't terrible that I wasn't really paying attention to how incredibly *quiet* this amp is.  This thing should be crushingly loud, and even dimed it's quite listenable... it doesn't even start to show up volume wise until i'm past 6, really...

I wonder if I have a crappy speaker connection...

[BobL]

So... wired the speakers straight to the amp to test... same issue.

Playing with knobs, it seems that the overall volume is VASTLY impacted by the level of the tone controls... so I have to turn the treble/bass up quite a bit and then I can start to get it pretty loud... but holy smokes is it ever bright.

Is that normal for this amp, or have I gone and done something wrong?

[BobL]

Did some internet poking around, looked at the Duncan tone stack calculator, and it would appear that this is normal operation for this amp.

So things are quiet (in a good way, as in no hum), and the sound isn't bad... it's just bright as bright can be, so I'd like to darken the sound.  I pulled the 47pf cap on the treble control, which seemed to brighten the amp (I thought it would darken it, as if I clipped the bright cap on the volume, but that seems to be the opposite case here.  Would increasing the value of that cap help darken the treble control without cutting volume, or would it just cut volume?

[phsyconoodler]

Leave the cap on the treble control. I don't find the low powered twin to be bright at all.Depends on the speakers and guitar used as well.
  My tele with custom shop nocaster pups is perfect.My strat is also perfect.
What speakers do you have?

[BobL]

Jensen C12Q

Tried both my '58RI les paul and my telecaster... both with wolfetone humbuckers, which I don't think of as being bright pickups or guitars.

[HotBluePlates]

... it's just bright as bright can be, so I'd like to darken the sound.  

You have a Presence control. Is it set to zero yet?

I have a 5F4 Super copy. I never set the Presence above zero because it was too much of the wrong kind of treble. Got harsh fast.

That said, a prominent Nashville engineer had 3 rules for getting a better guitar amp tone while recording:
1. Make it louder
2. Make it brighter
3. Make it louder and brighter

What you think is very bright when playing alone may not seem that way when recording o playing live. And what sounds righ & full when alone turns out to be pure mud when playing in a group.

...  I pulled the 47pf cap on the treble control, which seemed to brighten the amp (I thought it would darken it, as if I clipped the bright cap on the volume, but that seems to be the opposite case here.  ...

That could only happen if you had the cap connected to the wrong lugs of the volume pot originally.

If you make a bright cap bigger, it will stay just as bright but start to allow high-mids through as well. If the cap gets big enough (comparable to a coupling cap) it will simply bypass the volume control and won't seem to have much effect other than boogering the pot's taper.

...  This thing should be crushingly loud, and even dimed it's quite listenable...

No, it's a 2x 6L6 amp (if you have a 5E8A Twin). It's not a 4x 6L6 Twin Reverb or even the 5F8 high-power Twin.

If fact, other than the number of rectifier tubes and the size of the speakers (and number of input jacks per input triode), your amp is same-as my 5F4 Super. Loud enough to gig with, as long as you can tolerate a little output tube distortion.

I bet the bigger speakers weren't enough to differentiate the Twin from the Super, and prompted the bigger output stage for the Twin at the end of the tweed era.

[HotBluePlates]

Oh... and the tone controls are interactive enough that you might have to have the treble near-off and the bass near-full to get a noticeable tilt towards bassy-ness.

I don't have the amp in front of me know, but I know some of use with amps with this form of tone circuit made some cap-value changes to get the circuit to respond more predictably. My Super is in another state, so I can't help you there, but the Tone Stack Caluclator can show you the effect of a part change.

[BobL]

HotBluePlates - the cap I removed wasn't the bright cap on the volume, but a 47uf cap on the *treble*... which I've seen eliminated in a lot of schematics.

Anyway, so I've just had the speakers sitting out on my table while I'm waiting for a cabinet for this thing, and I figured that I wasn't getting a 'true' sound out of it and maybe I shouldn't mess with things too much until I have the whole thing put together.

For a little test, I plugged in the speakers from my AC30 (Celestion Blues), and the thing is definitely not too bright at all - sounds rich and full and nice... and loud as hell. 

The MV I put in here actually seems to sound good with this configuration, so I have high hopes for when I have everything built into the actual cab.  If the speakers are harsh then, I'll know that I should just get some nicer speakers.

[HotBluePlates]

HotBluePlates - the cap I removed wasn't the bright cap on the volume, but a 47uf cap on the *treble*... which I've seen eliminated in a lot of schematics.

This is a 5E8A, as indicated in your thread title?

If so, there isn't a 47uF, or 47pF, anywhere in the amp... so I'm not sure what you're referring to now. I see you mentioned "treble control" but the value threw me.

So would I be correct in assuming this was the cap tied to the cathode follower output (V3, pin 8) and one end of a 0.1uF cap? If that is the cap in question, put it back and get the correct 250pF cap (o at least a 220pF). The amp would react strangely with that particular value.

[sluckey]

If so, there isn't a 47uF, or 47pF, anywhere in the amp... so I'm not sure what you're referring to now. I see you mentioned "treble control" but the value threw me.

Look at the layout for the 5E8A. The 47pF is mounted directly to the treble pot.

Just another of the many errors between old Fender schematics and layouts.

[BobL]

Yeah, it's on the schematic for the Fender reissue, and the Mojo kit (which I started from) has a 47pf silver mica cap that ties the wiper to the right lug.  I have an original (maybe modified?) Fender layout online that also shows it...

[HotBluePlates]

If so, there isn't a 47uF, or 47pF, anywhere in the amp... so I'm not sure what you're referring to now. I see you mentioned "treble control" but the value threw me.

Look at the layout for the 5E8A. The 47pF is mounted directly to the treble pot.

Just another of the many errors between old Fender schematics and layouts.

Thanks! Didn't even see that!

Natually, I was looking at the 5E8-A Schematic, as the layouts feel slower to me for finding parts/understanding a circuit.

[sluckey]

I didn't look at the layout until 1 minute before my last post. I'm a 'schematic first' kinda guy too.

[g-man]

Deleted

[BobL]

I sure did, but I still appreciate the reply!

[BobL]

I got this guy all finished up.  I ended up keeping the presence on the rear and the MV, and I think it sounds pretty nice.  I was worried about noise, but it's pretty quiet on idle.  It seems to prefer my Les Paul over my Tele so far... could come around with some break in and a speaker replacement.

I am not liking the ceramic Jensen speakers... pretty harsh and overly bright, so I have a pair of Alnico Weber P12Q clones (12A125S is what I went with) on order.  I imagine they will warm things up a little and get me to where I want to be.  If I gave the ceramics some time, they would be better I'm sure, but... eh.

I lacquered the cab with a couple coats of cut down shellac, and then a topcoat of nitrocellulose lacquer I had sitting around... it looks pretty nice.

I'd probably do a few things differently next time, but... you know.  Hindsight and all.  I'll post a picture or two tomorrow if anyone cares to see.

[BobL]

Oh, and that presence knob is in the exact right spot to reach in the back of the amp and fiddle with it... though so far, as suspected, I'm running it all the way off.  There was maybe a bit more room inside the chassis if I'd put it on the power tube side of the output, but then I think you'd be burning your fingers on the power tube every time you reached in there, and the preamp tube there is shielded and not as toasty.

[Willabe]

I'll post a picture or two tomorrow if anyone cares to see.

Heck Yes we'd love to see!!!!!    

Very glad your build worked out for you!



                  Brad      

[ajeffcote]

Where's that picture?

[BobL]

I should do that today...

So I exchanged PMs about this amp with ajeffcote and I'll mention something here that I mentioned there:

Modern 12AX7s in this amp did not seem happy at all... boomy and harsh.  I put in the 12AY7s I got with it, and they sounded better, but pretty clean.

I now have some NOS 12AT7s in V1/V2, a 12AY7 in V3, and the 12AX7 in V4, and this seems to be the sweet spot tube wise... enough crunch, but a sweeter sound.  I am noticing some hiss, but it may be my house, because I'm getting weird sounds out of other speakers (maybe since the furnace started running?)  I had a 12AT7 in V1 and a 12AY7 in V2, and it basically murdered the bright channel - very quiet.  I should be able to run this combo safely though, right?

Still waiting on speakers.

[John]

I had a 12AT7 in V1 and a 12AY7 in V2, and it basically murdered the bright channel - very quiet.  I should be able to run this combo safely though, right?

Yes, but try swapping them around for fun (but I bet you already did). The AT7 is a much lower gain than the AY7, and V1 is only getting the signal from your pickups in the first gain stage. I have used the AT7 though in V1 position, parralled even, and running that into a 5879 and it sounds amazing *in that amp*. Reason I tried was an AX7 in that spot was just over the top, and it was easier to swap a tube than change resistors and stuff.

In my limited messing around on the breadboard, I think you can make any circuit sound good with any tube by tinkering with the other component values. But, when it's time to replace them....  But, unless you're really punishing them, I believe preamp tubes are supposed to last years and years, easily.

[BobL]

My impression is that the 12AT7 is higher gain than the 12AY7... or do I have that backwards?

I have swapped these around a bit, and I'll continue to experiment to see what else comes out of the amp, but... yeah, it's fun.

[John]

Testing my memory instead of constantly looking stuff up ;) AT7 is lower mu but higher current. I believe a lot of guy like 'em in the PI spot too.

But, I could be totally wrong.