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Post subject: SRRI Unequal Bias Current, one tube 27mA
Posted: Tue Jul 26, 2011 3:38 am
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Hello,

New here, hate to post this but my search turned nothing up despite this being a surely common problem.

Got a new Super Reverb Reissue. First Fender (or any other) tube amp. Long story short, due to my location, this had travel almost 12000 miles by autos and boats to get here (google "roi-namur" if you're curious). Plugged it in... Horrible wet two-stroke sound, breaking up at about 4 on the vol, not musical at all. I'm sure it got a good bump or three on the way here because the reverb tank had come loose from its screwed-down cardboard mounting.

(why does fender mount the verb tank on a strip of cardboard in a tolex bag? maybe there's a good reason? different topic... yes, I know you're listening)

I checked the bias, 33mA as measured at the left socket, 26.8mA on the right. That's almost a 20% difference. So... I swapped the tubes in their sockets, one for the other (same tubes, opposite sockets, NOT new tubes). The reduced DC quiescent current followed the tube, so now the left socket measures about 27mA. So...

This means I need new power tubes, right? A matched pair should not be different by twenty percent? I guess the other possibility is a problem with a resistor or diode in the grid supply or something, but the tube seems most likely, especially given the horrendous journey the thing had to take to get here.

Any thoughts would be appreciated, but absent any good suggestions otherwise, I'm going to try new tubes and I'll post further...

Thanks!
Steve


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Tue Jul 26, 2011 4:02 am
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(why does fender mount the verb tank on a strip of cardboard in a tolex bag? maybe there's a good reason? different topic... yes, I know you're listening) -sstepvep

Answer; without that mounting you could have feedback

You don't need perfectly matched pair . Matched pair could be unmusical, sound little bit sterile, but is a matter of taste . Difference between 5 to 8 milliamps between is good. That's what I like myself.

Output transformer primary could have not the same resistance / impedance so the load to each tube could be not the same as milliamps won't be the same .


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Tue Jul 26, 2011 7:05 am
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I agree with Stratele52 about the biasing. 20% difference is not unusual these days. Maybe pull and clean and spray contact cleaner in all of the sockets and tube pins (power and gain stage/PI). Corrosion is very possible on a long ship-bound journey.

In fact, it prolly won't hurt to use a LITTLE cleaner on the plugs (esp the ones connected to the reverb tank) and jacks.

Worth a try. Good luck!


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Tue Jul 26, 2011 8:16 am
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+1 to all responses thus far.

Given the circumstances involved in this amp's transit from Corona to owner, it wouldn't be inappropriate to disassemble the amp and inspect everything lest some relatively minor problem become a major issue. After all, it's a precision piece of electronic gear -- not a replacement fedora for Indiana Jones.

:mrgreen:

Arjay

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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Tue Jul 26, 2011 9:54 am
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Unless I misunderstood the OP, he swapped the tubes and the current reading stayed with the tubes, not the position, indicating that the tubes are mismatched and the amp circuit is OK. And, the idle is around 10 and 12 watts per tube which should sound horrible. Raising the idle of the lowest current tube to around 40 mA at the test point, despite the imbalance, may improve the sound dramatically. The mismatch may get worse as idle current is increased, so I would still get a new matched set of output tubes. I would set the lowest current tube to at least 40 mA for better sound. Setting the tubes to 33 mA at the test point will result in an idle wattage of about 12 watts per tube, (40%)........the horror.......the horror........ :idea

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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Tue Jul 26, 2011 10:46 am
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Agree with Shim, bias is too cold IMO, 40 mA should make it sing. If it doesn't help then look into a new pair of matched 6L6, I really like the TAD 6L6GC-STR in my SR with a bias around 40 mA.

http://thetubestore.com/tad6l6gc.html


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Fri Jul 29, 2011 2:14 am
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I tried rebiasing, no improvement.

The details: My plate voltage is 433V. Assuming the 6L6GC is rated for a max plate dissipation of 30W, and a useful bias range is 50% to 70% of max plate dissipation, then my bias range is about 35mA to about 48mA.

At the upper extreme, I was only able to get even the hotter tube up to about 40mA, and the slightly weaker tube to about 34mA. This is within the above range, but produced no improvement. Am I correct in thinking that I should be able to get more idle current out of both tubes?

New tubes on the way regardless.

I also tried reseating all the tubes, checked connections, looked for cold solder joints on the board, etc. Everything else looks good. I also tried replacing the 12AX7s one at a time, no change. A friend is bringing me a 12AT7 tomorrow to try just in case.

Further thoughts?

Thanks,
Steve


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Fri Jul 29, 2011 3:37 am
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At 433 volts at plate , I'll bias at 40 ma, not more, on the hotter tube. But once the bias is set to 40 ma re -check plate voltage and re bias if needed until no voltage change.

if I understand ; you wish your amp give more clean ? You don't want it break at 4 ! Right ?

You should have a 12AT7 on Phase Inverter, the tube close to the 6L6

This tube / circuit is very important to give clean or overdrive. If you have 12AX7 this will give you more distortion. The resistors in PI circuit should be have the right ohms too.


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Fri Jul 29, 2011 8:29 pm
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Votre Anglais est beaucoup plus meilleur gue mon Francais...

I believe it is a 12AT7 already, that's what comes stock on the 65 reissue, I think. I'll double check.

I don't need it clean all the way up to nine and a half, but four seems a little early. The king of clean, right? It really all depends on what I'm playing.

It's really more the nature of the sound. I am going to post a clip somewhere so folks can hear what I mean. But I'll have to do this later from my other, higher speed, connection...

thanks


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Sat Jul 30, 2011 1:53 am
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sstevep,

First of all, it s very hard to tell if an amp is normal or not if it break at 4.

1- What kind of guitar you use ? The pickups out put could give more or less signal. With humbucker the amp break earlier than single coil. I'm sure you know that.

2- Preamp tubes; all of the 12AX7 and 12AT7 could have more or less gain. You could have to try different brand. And also the 6L6 with a colder bias = more clean at high volume but could be thin a low volume if to cold.

12AX7 have gain of 100
5751 70
12AT7 60

3- Volume pot could have "wrong" resistance. I see that very often on new Fender pots from the store. I can't use some one. This could make very very little difference but with other out of spec components.......

4- On Phase Inverter circuit, an out of spec resistors or the positive feedback circuit from the speaker.

Last year I had to fix 2 Blackface Super Reverb '60 area , one is a lot more clean than the other and I never find ( in "short" time) what make this. Maybe everything have different spec from the other amp. Tubes, resistors transformer . Tubes are all NOS on the 2 amps.


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Sat Jul 30, 2011 9:47 am
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Interesting thread. The break-up at "4" is with the power tubes biased at 26.8 & 33mA? For such a conservative bias, in an amp noted for clean tone to very loud (the only one I've owned was an old BF)---- this seems a but odd.

And you can't bias the thing above 40mA, right? You peg the bias pot at this point, I'd suspect bad power tubes. You have a set of known good 6L6GC's that you can't sub into the amp. Be sure to turn the bias pot all the way CCW to "cold" setting, first. Then, crank it up to around 35mA and listen to the tone. This should be about half-way in the pot's range of motion. Maybe a little more, maybe a little less--- it varies from amp to amp.

One other side note. HOW are you checking the bias? I presume you are using a Tone King Bias tool or something similar. Have you tried biasing any other amps with this tool? Sometimes, the tool's one-ohm resistor(s) goes outta spec. Your true idle bias current could be a lot higher than what you are reading.

I've have several GT Bias Kits kinda fall apart (esp the early ones with poorly made socket adapters). Such that the contacts and plugs were no longer good. Started getting all kinds of funky reading.

Anyhow, with standard Fender Strat or Gibson LPS with not too hot pickups... and NO pedals in between--- your SRRI should have pretty clean tone to very loud.

Good luck with the amp.


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Sat Jul 30, 2011 1:21 pm
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+1 BMW for .."how are you checking bias ? "


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Sat Jul 30, 2011 3:26 pm
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Looks like the SRRI has two test points, one for each output tube. When the OP mentioned 33 mA, that is what the schematic lists for each test point, so I presumed that he was using the test point (Cathode current). But, each test point is Cathode current, so about 5 mA should be deducted to get the Plate current, meaning that 40 mA at the test point is only 35 mA Plate current. :idea:

http://support.fender.com/schematics/guitar_amplifiers/65_Super_Reverb_schematic.pdf

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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Mon Aug 01, 2011 1:28 am
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Wow, I'm starting to feel a little high-maintenance here... Thanks for all the attention!

Here's what I've got to check bias (I like this, and will recommend it pending verification of the 1Ω resistor value):

http://www.amp-head.com/product_info.ph ... ucts_id=31

So, to be clear, it's a one ohm resistor in series with the cathode that allows me to measure mV as a direct analog of mA. I have only made use of this in terms of DC bias. It would certainly be possible to view a waveform on an o-scope as well. This is a level of detail that I have as yet not aspired to in this process. Yet.

So, to be clear again, I was not using the test point shown in the amp schematic. I was measuring the cathode current at DC with no guitar input signal to the amp. Shimmilou, you have found that about 5mA is reasonable for the control- and screen-grid currents at a typical given DC bias? (cathode I - grid(s) I = plate I, yes) I am happy to bias according to my ear within the aforementioned acceptable range.

I am also awaiting one this same vendor's plate voltage divider sockets, as I enjoyed inserting my meter probe between a half-inserted tube and its socket to graze the pin even less than anticipated. Which is to say not at all, and I am by no means a stranger to high dc voltages.

I am awaiting a new set of power tubes to try. I also have a 12AT7 to try in those two slots, I'll post results. I will also be checking pots, and if they are all the correct value, I will likely be checking all the other component as I am able.

My primary guitar is a Schecter C1, two humbuckers. In my experience, their output is not unusually hot. Remember, in addition to the early breakup, I also find the nature of the tone to be less than musical. Adjectives that have come to mind include "two-stroke," "spitty," "splatty," and "whoopy cushion."

To that last part, I will be (finally) spending some time very soon recording a few examples of what I'm describing and posting them someplace online where y'all that have been nice enough to pay attention can check it out. I will post some recordings of the same guitar through other amps for comparison as well.

Thanks!
Steve


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Mon Aug 01, 2011 2:00 am
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You use a good bias probe, I have the same.

A guitar with humbucker have more output than single. This could be your breaking at 4. Can you test the amp with single coil guit ?
Or with volume on your Schecter not at 10 ?


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