high plate voltages in blues deville

[slapbackdaddy]

Hello again guys, sorry it took me so long to get around to checking out the problem after you kindly gave me some suggestions for my blues deville which was cutting out at higher volumes. Well, i did what you recommended, re-flowed the connections on the dropping resistors and pin sockets, everything else looked fine on the board. I have a slightly louder hum on the normal channel than i do on the drive channel which seems to benefit from a fresh pair of tubes in V1 + V2.....it's better but still there though.

I took some readings from the output tubes that seem too high to me, what do you think?

Pin 3 / plate = 509 and 510vdc
Pin 4 / screen = 509 and 509vdc
centre tap of OT to pin chassis = 514vdc

Am i right in thinking that these Electro Harmonix 6L6GC tubes are rated for a max of 500vdc on the plate and 450vdc on the screen?

The amp is 1993 fender blues deville, 14 years old, am i getting the extra voltage from leaking filter caps? I've ordered replacement caps anyway and will replace them. Anywhere else i should be looking to account for the voltage readings?

Thanks in advance.

[MWJB]

They seem normal, there's always a bit of leeway based on how much plate current the 6L6s are drawing.

I very much doubt that the EH 6L6s are only rated for 450v on the screens, most 6L6 Fenders run more than that. Therefore, such tubes would be useless in the real world. Sure you're not thinking of the EH 6V6, those ratings are more in line for a modern production 6V6?

Many 6L6 amps run in excess of 500vdc on the power tube plates, I wouldn't be concerned unless you were getting well over 525v. I doubt any of your problems are directy attributable to the B+ voltage.

[slapbackdaddy]

OK cool, puts my mind at rest. These are 6L6GC tubes. I was reading the 6L6GC specs on Ampage:
http://www.firebottle.com/ampage/td/vtd6l6.html

and noted the maximum values for use were 500vdc on the plates and 450vdc on the screens....that why i thought my readings were too high...they were over maximum.

If you say most fender 6L6 amps amps run with these voltages then maybe that's fine, i appreciate your expertise with this. I just needed to be sure that they weren't or won't get fried....

[MWJB]

Keep the current reasonable and they'll be fine, see what they sound like at 30mA.

Tolex Fenders typically run the screens within a couple of volts of the plate (SF Champs & Princetons can have a higher screen voltage than the plate).

[slapbackdaddy]

Originally Posted by MWJB

Keep the current reasonable and they'll be fine, see what they sound like at 30mA.

Tolex Fenders typically run the screens within a couple of volts of the plate (SF Champs & Princetons can have a higher screen voltage than the plate).

I'd need to install a bias pot for that though wouldn't i? No adjustment on these babies.

[MWJB]

Replace R87 with a 50KL cermet trim pot, wired as a variable resistor, (or, if room, a 50KL trim pot with a 22K fixed resistor to ground - the resistor will save you from putting hundreds of mA at idle through the tubes if careless adjusting), set to 27K before firing up, then power up with no power tubes fitted, adjust pot to give you -55v at pin 5. Power down, fit tubes & bias to 30mA.

Or, if you have access to a bunch of matched pairs of tubes, keep subbing until you find a pair that runs, say, 28-33mA.

[slapbackdaddy]

Thanks again, will fit the trim pot for bias adjustment, just need to track one down and order it (i'm in the UK). Great advice as always.

I'd like to reduce the headroom if possible and bring some earlier breakup, i use lower output old single coil guitars like Harmonys and silvertones and don't want a wall of rock sound just some more sizzle at lower volumes. Just wondering if anyone had biased a deville or similar for that sort of sound and what numbers they aimed for....you mention biasing to 30ma, i'm presuming that's a good mid-ground setting. The pair of electro harmonix 6L6gc's that i just fitted are matched to 42mv...higher than you suggested but that's what i have. What sould i be aiming for roughly to drive things a little easier?

[MWJB]

RSWWW.com. You should be able to find a 750mW long, multiturn, rectangular cermet pot with the adjuster on the end. Easiest type to fit to crowded PCB. Glue pot in place with clear silicon cement (Loctite 5145, also available from RS).

42mV is hotter than I would run at 510v on the plates. I would lose 10mV if I were you.

Biasing is not really the best route to tweaking power output.

[slapbackdaddy]

So if the 42mv tubes are too hot to run at 510v on the plates i either get some more tubes or fit the bias trim?....i think i'd like the control in the long run so i'm gonna go with the trim pot and adjust things that way. It's a learning curve for me so thanks for your experience here. I will probably be back to bug you again soon!

[MWJB]

You refer to them as "42mV tubes", they are actually reading 42mV whilst in your amp aren't they? It's not because they have 42mV written on them by the vendor? They are only 42mV tubes if that's what you read, via 1ohm cathode resistors (or probes fitted with such - how are you measuring current at the moment?), whilst in YOUR amp.

If you are reading 42mV in your amp - EH6L6s are not, generally speaking, particularly hot running tubes, so fitting the bias pot is the best route.

[slapbackdaddy]

Thanks again MJWB,

Well, you're right, the tubes are marked as 42mv on the tube boxes.....you made me realise that it won't be the case in the amp! Anyway, lesson learned! Now, i need to measure the cathode current and i don't have one of them bias probes, and don't have a 1ohm resistor handy to ground off the cathode (that's where you take the reading....right?)....is there another way involving a DMM and calcutator that i can take a current reading without having to order some resistors? (there's nowhere locally i can get components).

[MWJB]

You can measure the plate current with just a DMM AS LONG AS YOU HAVE CLIPS/WIRE GRABBERS! Don't try this method unless you have the means to secure the DMM leads to the components mentioned on the amp - you will be dealing with the highest, MOST DANGEROUS, voltages present in the amp! Do not attempt to hold meter leads in place.

1) Turn the amp off & unplug from the wall.

2) Clip your red DMM lead to "CP3", this is where a thick red wire, from the output transformer, connects to the PCB.

3) Clip your black DMM wire to pin 3 (clockwise from the socket keyway, looking from the top of the socket) of the 6L6 you are measuring. If I remember right the BDV has metal wire/struts running from the actual tube sockets to the 6L6 PCB, this will be the easiest place to connect the meter. Triple check you have the right pin, pin3 will also connect to CP1 or CP2, depending on which tube you are looking at, these are where the brown & blue OT wires terminate.

4) Set meter to <200mA DC.

5) Check again that all is secure, that nothing can fall off or short.

6) Power up the amp. The reading showing on the DMM is your plate current in mA. Give in a couple of minutes to calm down.

7) Power down, unplug from wall AC and move black DMM lead to pin 3 of the other power tube & repeat procedure. When finished, power down again reset meter to volts, to save you blowing it when trying to read voltages later.

You want to see 28-33mA for each tube. They can be +/- 5mA and still be a matched set. Report back.

If you don't think that you can follow these instructions exactly, take the easy route and buy some bias probes, TAD can get them to you within a week. If anything is not exactly as I describe, stop & query it here.

[MWJB]

"(there's nowhere locally i can get components)." Whereabouts are you? RS & Maplin deliver by post, Torres in Pinner (0208 933 0918) carry many common amp parts, as do Bluebell in Dundee (01382 322 990).

[slapbackdaddy]

Thanks for that, i'll report back shortly with the measurements. I've
re-capped the power supply electrolytics and should have had the bias caps today in the post (next day delivery) but they didn't arrive....great. Have also got a 50k cermet trim pot and 22k resistors coming too so will have to fit that lot after the weekend as i have gigs Fri/Sat/Sun. Have just powered up after fitting caps, will take some measurements in while and get back to you.

Yeah, everything is mail order for me, i'm aware of those suppiers, i just didn't have the resistors to hand.

[slapbackdaddy]

OK, just after i sent the last post the other caps and stuff turned up. I've replace the bias caps also now AND fitted the 50k trim pot with 22k resistor....set it to 27k, removed the power tubes, powered up, took the reading at pin 5 and adjusted it to -55v. Powered down, re-fitted the tubes, clipped red lead of DMM to CP3 (do i completely remove the red lead or do i squeeze lead and croc clip onto same terminal???....i had them both on there), black lead went to pin 3, DMM set to 200ma DC, powered up.....the reading is 0.00 with a very quick flash of 0.02 on full power then going back to 0.00. Am i doing something wrong here? I can't get a reading at all on any of the DCA settings on the DMM. What next?????
I'm DEFINITELY on CP3 - red and DEFINITELY going to pin 3 of the 6L6....it's the same way i took voltage readings originally. Help!

[MWJB]

Red DMM lead & red wire from OT (the centre tap) both need to be on CP3 at the same time.

Check the mA fuse in the DMM.

"it's the same way I took voltage readings originally"...surely when you took readings before the black DMM lead was grounded, not hooked up to pin 3?

[slapbackdaddy]

Yep, fuse gone, changed that and am getting readings now. What i meant about previous testing but didn't make it clear enough when re-reading my post was that i was aware of CP3, and was sure i had the right terminal....yes, i was gounded before on the previous tests.

OK, first readings were LOW, around 7ma, so have wound up the trim pot to give me roughly 30ma on both tubes.

If i use webers bias calculator and input my current plate voltage (515v) it gives a bias setting 40.7ma for 6L6GC tubes to run at 70% plate dissipation.

You suggested 28-33ma....is it just a case of taste now, listening to what seems better or should i be heading more towards 40ma....or not!?

Thanks a million for all your help so far MWJB, you've been a star!

[MWJB]

EH claim 30W for their 6L6...tube vendors "in the know" have intimated to me that you are better off assuming 25W, which puts your 70% at 33.9mA. 30mA should be plenty to get a regular, Fendery tone.

JJ 6L6s could probably tolerate 40mA.

More current = more grease & more heat.

You're welcome, glad to be of help.

[slapbackdaddy]

I settled on 33.9 on one tube and 32.7 on the other....pretty good match i reckon...but then what do i know! Will see how it sounds over the weekend when i can give it some proper volume.

Thanks for talking me through the whole thing....boy, i've learned A LOT! + having the bias trim will help in the future i'm sure.

[Enzo]

Why make things difficult. The amp ALREADY has a 1 ohm resistor in the cathode leads of the power tubes. It is R66, and is right in the corner of the tube socket board. Looking in the rear of the amp, should be in the near left corner of the circuit board. Same place as the test point for the Hot Rod amps. Ground the meter, and then probe the end of the resistor right in the extreme corner.

[slapbackdaddy]

Thanks Enzo

Sounds an even easier way to do things!....where were you yesterday?

Anyway, everything is fine now, thanks to MWJB.

Cheers!

[MWJB]

Didn't think the '93 BDV had bias test point, thought that came in later (not on the '93 schematic)? Still, the OT shunt method allows you to see how closely matched the tubes are.

[Enzo]

OOOOPS. My apology. I was looking at the reissue - the current version from 2004. No pun intended. The original 1993 did not indeed have the resistor.

My disk has the two versions in two different file folders. Sorry.

I take nothing away from the shunt method, I use it myself, but in the presence of the little test point, I go for convenience.