Checking Fender Blues Junior Bias

[g1]

Please do not use the shunt method. It is far too dangerous for a novice. For that matter, don't even use a meter to directly measure current until you know exactly what you are doing. In current mode, your test leads become like a straight piece of wire. Sparks will fly unless you break into the circuit and put the meter leads in series.

The Blues Jr. and a couple others in the same series have a peculiar problem. Due to the layout, measuring voltage at the power tube plates can cause oscillation and give wrong readings. Some get around this issue by checking one tube at a time in the socket that does not oscillate, some get around the problem by using shunt method.
The simplest solution was posted by Dave H, just pull the PI tube. (preamp tube closest to power tubes).
Now with the preamp tube removed, proceed as outlined in posts #6,7, & 8.

[dadroadie]

Thanks for the reply G1!

I’m going to follow your advice and not use the shunt method. I just want to be sure I have the steps correct. If I’m wrong on anything please let me know. I don’t want to smoke the amp or myself.

1: zero the volt meter in the ohms scale and write down any residual reading for reference.

2: with the amp unplugged measure resistance between output transformer center tap (red) and each plate (brown and blue). Write down these measurements.
3: Turn amp on and warmed it up for 10 minutes or so, pull the PI tube. (preamp tube closest to power tubes), measure the DC voltage in the same locations. Write these voltages down.

4: Use ohms law to calculate milliamps: Ohms Law gives you current (I = V/R) Convert amps to milliamps, multiply that figure by 1000 for milliamps. Write down these measurements. I should see the factory bias voltage of around 35 to 40 mA sometimes more using the ohms law calculation.

5: Install a new set of Groove Tubes with the same rating as the old tubes (#4) and repeat the procedure. If I get the same or a little lower MA readings they should work like the old units. The original Grooves Tubes EL84s are marked 4 and are not showing any signs of overheating or red plating.

6: If I install the bias pot as was discussed adjust the new tubes to about 26 to 29 milliamps, That will give you good tone and maximum lifetime. Also if I do the bias adjust to this range could I use another brand like of tubes or go to a Groove Tubes #5 for a little cleaner headroom ?

Again Thanks to everyone for the help!
dadroadie.

[Enzo]

Um, it sounds like you plan to take a voltage reading at the red, and then at the blue. Really easier if you just measure the voltage from Red to blue. No math. You already measured the resistance, so Ohm's Law gives the current.

[Leo_Gnardo]

if I do the bias adjust could I use another brand of tubes and just set the bias to the 26 to 29 milliamps? possible go to a #5 for a little cleaner headroom or Tung Sols ?

Yes, you could adjust bias on any brand EL84. FWIW I've been having good luck last 10 years or so with JJ. If you have the option to order tubes with a generally low emissivity (Pc) do so. I've been getting mine from Antique - tubesandmore.com . Most gigging musicians around here get two years or more sometimes 5 or more. It depends on how much time the amp is on, how loud you're playing it, and how badly it gets rattled around in transit. Good to carry it on a car seat whilst driving, or put a cushion under it if it has to go in the trunk. One of my customers uses a dog bed, perfect for the purpose. Any old discard sofa or chair cushion would suffice.

Chicago - I have heard of one outfit there that charges gold dust money for service. I guess their competition figured, if they do it, we can too. OW! All the more reason to service your own gear. Plus, there's the general trend of inflation. Really has gotten to the point us geezers flinch. See this $50 bill? It's the new ten... Now I know how my grandparents must have felt.

Headroom - not much of a point on Blues Jr. I've seen power vary 13 to 15 watts at clip. Trying to panel-beat more wattage from this amp is an exercise in frustration. You would have more success with a super high efficiency speaker. For a long time Fender have been shipping them with Jensen C12N which is a perfectly acceptable speaker, with a power rating such that you'll never kill it with your Blues Jr. I'm sure some MEFsters could sound off here with their experiences subbing in other speakers.

Finally, a word about Groove Tubes. GT do not make tubes, they buy crates full of Sovtek/EH, quickly test & roughly match them for emissivity, and paint their logo on. GT has for over 10 years been a fully owned subsidiary of Fender. So it's all about making some loot - isn't it always - notice all Fender amps have been issued with GT for a long time now. Tubes marketed currently as Sovtek, ElectroHarmonix, TungSol, Mullard, and Genelex Gold Lion all come from the same outfit (New Sensor), sold at different price points. Some may debate me but I swear they're all the same tube from the same Saratov Russia assembly line, with different paint depending on how much profit they want to make. JJ at least are a totally separate brand, made in Slovakia. Chinese (Shuguang) EL84 marketed under several brand names - I call them Chinese firecrackers - maybe there's a reason for that. Wouldn't recommend them for use in any amp that puts more than say 275V on the plate. And now you'll be paying an extra 25% for them due to new import tariffs, what a treat! Phooey.

[SoulFetish]

Chicago - I have heard of one outfit there that charges gold dust money for service. I guess their competition figured, if they do it, we can too. OW! All the more reason to service your own gear. Plus, there's the general trend of inflation. Really has gotten to the point us geezers flinch. See this $50 bill? It's the new ten... Now I know how my grandparents must have felt.

I don't know, Leo. There are so many factors which affect the cost of repair (which I know you know). But I would think that the cost of real estate is probably the greatest determining factor. The cost of rent near our location just outside Boston is really expensive. But it is a great location, with pretty easy access for musicians living in the city of Boston, Cambridge, and all of Metro West. Between me and you, I don't think we charge enough money. Our service record is great, my boss has built a great brand and reputation over 30+ years, and he obviously only hires the most qualified lead technician in all of New England But that's beyond my purview, I don't set the price. I can only offer my unsolicited opinion and do what I'm told.
My friend once told me, "If you're not loosing customers, you're not charging enough money". So when I am king...

[Leo_Gnardo]

I don't know, Leo. There are so many factors which affect the cost of repair (which I know you know). But I would think that the cost of real estate is probably the greatest determining factor. The cost of rent near our location just outside Boston is really expensive. But it is a great location, with pretty easy access for musicians living in the city of Boston, Cambridge, and all of Metro West. Between me and you, I don't think we charge enough money. Our service record is great, my boss has built a great brand and reputation over 30+ years, and he obviously only hires the most qualified lead technician in all of New England But that's beyond my purview, I don't set the price. I can only offer my unsolicited opinion and do what I'm told.
My friend once told me, "If you're not loosing customers, you're not charging enough money". So when I am king...

Good observations SF. And I'd say we're wandering a bit from the OP's quest, BUT he did mention early on what a local tech asked for service. Fifty bucks for a pair of EL84, plus a C-note for a look-at, still kinda pricey no matter where. New York, London, Paris, Munich, who can afford to play rock music? BTW somehow I thought you were more west in Mass maybe Woosta or Springfield. Boston metro area, sure, the musos will keep you hoppin'! In my case some customers flee NY city & nearby suburbs for a day in the country (country - that's debatable...) to haul their demised amps to me. Cheaper prices and more thorough care too. Not the NYC quick & dirty dustoff & gimme your wallet hustle. So, from one end of the telescope, "If you're not losing customers, you're not charging enough money" and from the other end, "geeze life is getting SO expensive." For instance, I'm astonished at the average $100/hour rate auto mechanics charge around here. Also the pair of $425,000 houses that just went up on a nearby street, same size as mine. Will my property tax bill go up 250% now??? Crikeys!

OK USA techs, one day of holiday then back on your heads! Me, I'm gonna wrench an MV Twin back into health, then go have some barbeque with old friends.

[dadroadie]

Hi and thanks to all again those helping with this.
Just to set the record straight I didn’t say that what they are asking to do the mod is wrong. Their labor rate must be what the market will bear. Otherwise they would not be in business. I just can’t afford this. (Retired). I live in a sleepy blue collar suburb (about 40 miles outside of the city) but the service centers are located in towards Chicago. There are none in my area. Plus I really think I can do this thanks to all of the great folks walking me through this!

I wanted to post the procedure one more time so I’m sure I have the procedures correct. I’m going to order the trim pot and talk to the guys at Antique Electronics about tubes. I’ll let everyone know what I find.

1: zero the volt meter in the ohms scale and write down any residual reading for reference.

2: with the amp unplugged measure resistance between output transformer center tap (red) and each plate (brown and blue). leads still connected in the curcuit Write down these measurements.

3: Turn amp on and warmed it up for 10 minutes or so, pull the PI tube. (preamp tube closest to power tubes), measure the DC voltages resistance between output transformer center tap (red) and each plate (brown and blue). leads connected in curcuit .Write down these voltages.

4: Use ohms law to calculate milliamps: Ohms Law gives you current (I = V/R) Convert amps to milliamps, multiply that figure by 1000 for milliamps. Write down these measurements. I should see the factory bias voltage of around 35 to 40 mA sometimes more using the ohms law calculation.

5: Install a new set of Groove Tubes with the same rating as the old tubes (#4) and repeat steps 1 through 4. If I get the same or a little lower MA readings they should work like the old units. The original Grooves Tubes EL84s are marked 4 and are not showing any signs of overheating or red plating.

6: If I install the trim pot (schematic I posted,( you want to install the 10K trimmer in series with R52, the 22K resistor.) I can then adjust the new tubes to about 26 to 29 milliamps, that will give you good tone and maximum lifetime. Also if I do the bias adjust to this range could I use another brand like of tubes or go to a Groove Tubes #5 for a little cleaner headroom .

Again not trying to be crazy on this. Just want to be sure I have it right. Also I might help others to understand how the Blues Junior works
Thanks to All
dadroadie

[Dave H]

3: Turn amp on and warmed it up for 10 minutes or so, pull the PI tube. (preamp tube closest to power tubes), measure the DC voltages resistance between output transformer center tap (red) and each plate (brown and blue). leads connected in curcuit .Write down these voltages.

I'd pull the PI tube before turning the amp on.

[Leo_Gnardo]

I'd pull the PI tube before turning the amp on.

Yes - I was gonna say....

As for Groove vs anyone else, GT does not publish a guide of what their numbers mean. Same thing at Mesa, their tube matches bear color codes, what they mean is a mystery. One advantage of ordering from Antique or any other outfit that actually reports real emissivity and gain figures is you get closer matched tubes, and in the future you can request same or similar tubes with a reasonable expectation you'll get them.

One thing about chatting with Antique, understand they're not going to give you a whole lot of phone or email time over a low buck deal. If you're ordering hundreds & 1000's of $$$ worth, then maybe. They have a business to run.

Headroom again... a popular buzzword but what does it mean? From what I gather, more headroom means being able to dial the amp past whatever point you're at, and still sound clean. To do that takes power, and in an amp that barely makes 15 watts, there's nowhere to extract more power. Earlier I suggested a possible speaker change. Speakers are rated by sensitivity, how loud they get with a specific amount of power, say 1 watt measured at 1 meter distance. AND that's just at one frequency. Manufacturers have been more forthcoming in recent years, but a comparison of efficiency figures can still be misleading - say you have one speaker that sounds very bright and you compare it to another that doesn't accentuate high frequencies so much. They might have near identical general sensitivity figures but subjectively the brighter one will catch your attention as being louder. That's why I called for our MEF friends to share their experiences swapping out speakers in a Blues Jr, so you can get some idea what might satisfy without having to go through a long, expensive process trying out speakers.

Another headroom factor - is the amp loud enough, but you get into the crunch zone a bit too quickly when dialing up the volume? Consider swapping in a preamp tube that has less gain, say a 5751 or 12AY7 in the first preamp tube socket. You can order one or both from Antique or wherever you decide to buy EL84's. No adjustment needs to be made to the circuit. It's a cheap experiment, under $20 a tube, and might get you where you want to be with no further changes. As an example, a couple weeks ago I worked on a Princeton Reverb for a fellow who plays surf music. These days you can't haul a Twin Reverb into a club and dial it up loud & clean unless you want to be shown out the door quickly and permanently. So - to get his "loud and clean" requirement with his 15 watt Princeton, 12AY7 pre tube and Bob's your uncle. He can dial it way up, stay in the clean zone & evvabody's happy. If more volume is needed, put a mic on the amp & leak a little of it into the PA. Happy musos, happy club manager, happy listeners & dancers, what more could ya want?