dumby tube, using a resistor

[shooter]

Hey, I let the smoke outta my cathode bypass cap so I figure I probably outta ask some questions before I do that again!   I’m using the GA-8 as the basic build but using 6L6’s.  My PT is 380-0-380 at the rectifier plates n about 540vdc at the first filter cap/resistor point with only the rectifier tube in.  what I’d like to do is substitute a resister , plate to cathode to “load” the supply so I can set the plate cathode voltages n current.
Wondering what a tubes dc resistance is, the data sheets I’ve got don’t give me that info.

Thanks for the help.

dave

[stratele52]

If I understand your question , you want to put a resistor to simulate tube's plate/ cathode load .

6L6 at 540 volts use about 35 milliamp for an average bias. We can call that the current flow bewtween Cathode to Plate  to make a short answer.

The resistor you need will be;   540 v / 35 ma = 15. 4 k ohms

Power of the resistor ; 540 X 35 ma =  19 watts , This the watt dissipation. If you want a resistor that wil do the job for a couple of hours . I'll use at least  3 X 19 watts  = 60 watts resistor.

[DummyLoad]

Hey, I let the smoke outta my cathode bypass cap so I figure I probably outta ask some questions before I do that again!   I’m using the GA-8 as the basic build but using 6L6’s.  My PT is 380-0-380 at the rectifier plates n about 540vdc at the first filter cap/resistor point with only the rectifier tube in.  what I’d like to do is substitute a resister , plate to cathode to “load” the supply so I can set the plate cathode voltages n current.
Wondering what a tubes dc resistance is, the data sheets I’ve got don’t give me that info.

Thanks for the help.

dave

what was/is the voltage rating of the cap that fried and what is the voltage at the cathodes with respect to ground? you don't need to simulate the load - use the resistor you have without the cap installed.

fire up the amp and measure the voltage across the cathode resistor. take that reading and add 20-25% to it. get a cap with closest standard voltage rating at or above the 20-25% margin.

if you would; please post the loaded B+ measurements (all the tubes installed and lit) the cathode resistor value, voltage across the cathode resistor, and the resistor's wattage rating. the OT primary impedance rating would also be helpful. 

--DL

[PRR]

> I let the smoke outta my cathode bypass cap

Polarity ? ? ?

You don't need the cap to bias-up the amp. It may (and may not) improve the sound at FULL power.

If the cap blew from over-voltage, check that you have less than 1Meg (should be 220K) from each pin-2 Grid to ground.

http://www.gibson.com/Files/schematics/GA-8.PDF

This is a 6BQ5(EL84) amp. If you think to run 6L6, you need to tell us a lot more.

Clearly 2-6L6 will need a very different bias resistor!!

For the 6BQ5 flavor:

The nominal B+ is 283V. The cathode bias shows 9.3V in 130 ohms. 9.3V/130r= 0.072A. The other stuff drops 40V in 22K, call it 2mA, total load 0.074A.

We need a resistor of 283V 0.074A. 283V/0.074A= 3,824 ohms. 283V * 0.074A= 21 Watts.

Two 2K 10W parts in series is pretty close and will stand such abuse for a minute at a time, long enough to read voltages. For long-term stress-testing, figure four 10W resistors and give them plenty of air on a flame-resistant board protected from fingers and papers.

Three 10-Watt 120V dryer-lamps (Home Depot has them) in series is another option. They won't glow full bright and the effective resistance is dubious, but it's in the right ballpark, will stand the power, and gives clear indication that power is ON. (I'm not sure H-D has the sockets.)

[shooter]

What i'm trying to do is load the power rail without the tubes cuz i'm useing a freebee PT which is hoter than the one the gibson was designed at.  so I need to re-do the power rail to get close to current and plate volts b4 i let smoke outta the tubes:)  so what i'm trying to find is a way to simulate actual tube voltage drops/ current flow.  I'm thinking B+ @ 400ish and a current of 80mAish measured into the OT primary from the power rail.  the smoke came outta the bypass cap cuz i used a 100w 1 ohm "load" which because i was being stupid put 500VDC on my poor 35vdc 50uF cap! btw, all my fingers were in my pockets!!

[Willabe]

You said right now you have 540dcv at 1'st filter cap B+ node? You want to get it down to 400dcv?

140dcv at ~40 to ~50mA is a lot to drop with a R.

IMO it would be way easier to drop the dcv with Power Scaling (VVR). And because your amp is cathode bias you can use the less expensive and less complex version. (No grid bias tracking needed.)


                 Brad       

[PRR]

Before you get too far, it would be good to know:

the OT primary impedance rating would also be helpful.

[shooter]

I did some hunting on VVR n that does look like my answer.  I could also buy a more matched PT n save this tranny for a bigger build.  Someone asked about the OT impedence, i'm using hammonds GSE, with 2 secondary 1/4" jacks. one labeled 8ohm - 6v6, the other 4ohm 6l6.  both jacks use the same yellow/black wire.  The tube data sheet for 6L6 says 500vdc is max plate voltage in pentode mode, i've scaled it down to 470vdc with about 330ohms @20watts in the power rail and got my 12ax7's volts to about 197vdc by adding a 47k 3w in place of the 22k.  i'm thinkin that should be good enough for a first round "sound-check".  in the tube data sheets there a spec "plate resistance, apoximate  33k" for the 6L6.  fixin hi-end electronics for 30yrs, i'm pretty good at, but i've always left the real engineering to the guys that can't fix shit :)  Now i'm trying both!  does anyone know what that value represents?  Is that the aprox resistance the tube presents in the dc series path of the tube?  anyway, thanks again, this is a great school!

[jazbo8]

First, the definition of plate resistance: https://dl.dropbox.com/u/1326040/Plate%20Resistance.gif.

Please note that the 33k example that you sited, is specifically for the conditions shown in the datasheet - it is not a constant, i.e., if you change either the plate voltage or current (or screen voltage as well), the plate resistance will change.

Another point, you seem to be mixing maximum value of the tube with the actual (or desired) operating condition, "by knocking it down to 470V" as you indicated - it is still far away from examples given for the typical operation of the 6L6. If you want to do a test, you should do it close to the actual operating conditions. I think the voltage drop required in this case, is pretty large and a bit of a waste (even if the power resistors are relatively cheap), so getting a more suitable transformer might be a better way to go.

Jaz

[PRR]

> "plate resistance, apoximate  33k"

Plate resistance of a pentode is *NOT* the DC Voltge over Current.

Pentode plate resistance is under 1K for plate voltage up to about 20% of G2 voltage, then over 10K for higher plate voltage. For 6L6, more like 800 ohms and 33K. The DC equvalent will be somewhere between. "Between 800r and 33K" doesn't tell you much.

> hammonds GSE

125GSE is single-ended. GA-8 is push-pull.

I also fix sewer pipes. Single-ended, not push-pull.

[shooter]

Thanks for the link jasbo8, that was what i was thinkin as far as design goes.  I did tube it up and they loaded the B+ rail pretty close to the dummy resistors I used, i wound up with about 350vdc at plate and about 270 at screen, 18vdc on cathode.  still gotta do current checks but  i have a problem with the preamp section i'm tryin to track down now.  hopefully that'll get me to my second sucessful build.  Thanks again for all the help.  I do have an unrelated question on audiophile builds; Which is the preferred amp, class A or class AB? (not factoring cost, weight, efficency), just sound quality, reproducability sorta thing.

thanks
dave

[jazbo8]

Audiophile builds are usually Class A, of course not always though, many times it is not so much about the topology but the components selection, circuit layout and chassis design.

Jaz

[HotBluePlates]

I do have an unrelated question on audiophile builds; Which is the preferred amp, class A or class AB? (not factoring cost, weight, efficency) ...

Do you really want to chase audiophile desires? Then you need to figure out what gives your customer the biggest woody. And it will change with every customer.

The preferred amp depends more on marketing, fashion and whim more than any technical consideration. The audiophile market takes technical information regarding small improvements of known circuits, applies ultra-hype, then sells a product with the tweak at a 1000% upcharge (or more) over the stock item. Most of it is selling someone a fantasy, or a feeling of excitement.

A few years ago, that whim went for directly-heated triodes (like the 300B or 2A3) often in a single-ended arrangement. Look up the prices for NOS 2A3's, 845's or Western Electric 300B's. Cost is off the chart, output power is often in the single-digits, and you need some good horns to get usable volumes with the available power.

The cost just to build and audition a prototype is rediculous. It also sucks to use an old power triode that needs a driver with 400v+ of B+ to get a big enough signal to drive an output tube running on 250v B+. You'll need more than double the signal size that you might use to drive the output tubes of the biggest, baddest pentode guitar amp output stage.

[kagliostro]

For me audiophiles are widely damaging the valve market

and that without that there is a reasonable reason

 

K

[shooter]

My second build works!!  I made the final solder connection, pluged in a guitar, played for 10 minutes then my son calls n says "i'm 50 miles away, i forgot the amp you built, is the other one done?!!  road trip!  he says it's just the ticket after a 4hr session.  Next.....   No, the "audiophile" build is/was for me but i priced out the iron X 2, the kt88's from St. pete X 2 n all the "cheap" parts n i'm WAY over budget!  maybe a stereo el84 with premium "cheap" parts!

thanks again for all your help

dave