7591 conversion to 6V6

[oldhippy]

I'm starting a new project here and I could use some input. I came across an old Crown bass amp, using a pair of 7591's. It is a poorly built thing that closely resembles an Elk amp. I plan on gutting it out and building a TOS out of it. I'd like to use the OT that it came with, if practical. The effective output R of a pair of fixed bias 7591's is listed as 6600 ohms, fixed bias. I'd like to use 6V6's or 6BQ5's, cathode biased. Load R for either of these is listed as 8000 ohms. Is this close enough? Or should I pop for another OT? Other alternatives? Thanks, guys...

[gldtp99]

Use a pair of 6L6GC's and you're there.......................gldtp99

[Dave]

6L6's pull a little bit more heater current than 7591's. Probably not enough to make a huge difference. 6V6's pull less.
 
Dave

[tubenit]

Hey, IF you use a 360 ohm 10 watt resistor ............... you might be able to  switch between 6L6, 5881 or 6V6 tubes.  Both my TOS and TBM can use 6V6 or 5881 or 6L6 with no bias change at all.

I personally am not real keen on the EL84 tubes. I have yet to hang on to an amp that had EL84/6BQ5 tubes.  Not sure how they would sound
with the TOS?   

Keep in mind that ALOT of the overdriven  tone for the TOS is in the preamp and OD section.  So switching to EL84's may give you a somewhat more distorted (vs. overdriven) tone?   

The other issue is the EL84's are more limited on the voltage to the plates which could be problematic getting the preamp and OD tube plates with enough voltage.  Hope that comment makes sense to you? 

To determine if your PT can handle the 6L6's, plug them in and measure the heater voltage. Does it drop more voltage  than when 6V6's are
plugged in?   If not, you might be OK with 6L6's.

What is the voltage of your PT?  Is it close to 300-0-300 or so?   And does your PT use a solid state rectifier or tube rectifier?

With respect, Tubenit

[pullshocks]

7591's sound pretty good in their own right. 

[Dave]

+1 on that. 7591's are great tubes IMHO. A TOS with them might work out to be a unique personalization of the amp for you.

Dave

[tubenit]

+1 on that. 7591's are great tubes IMHO. A TOS with them might work out to be a unique personalization of the amp for you.

Excellent point!

With respect, Tubenit

[Tiny_Daddy]

I have been converting Blues Jr. to use JJ 7591 with good results.

[gldtp99]

sorry--- my apologies....................gldtp99

[Willabe]

I guess the 0.9A heater current spec of the 6L6GC vs the 0.8A spec of the 7591 means that the common 6L6GC tube isn't suitable

I don't think that 0.1A (per tube) is going to bother the PT, unless it's already over stressed/maxed out. A 12A_7 is 0.3A (300mA) so if it's single ended amp, your only adding 1/3 of 1 pre amp tubes filament current draw.

Where you can get in trouble is when you go from a 6V6 (0.45A) to 6L6 (0.9A), or 6L6 to EL34 (1.5A) or 6550 (1.6A), now you've doubled the current draw.

To determine if your PT can handle the 6L6's, plug them in and measure the heater voltage. Does it drop more voltage  than when 6V6's are
plugged in?   If not, you might be OK with 6L6's.

               Brad      

[Dave]

Hatchet buried.

Dave

[HotBluePlates]

The significant difference (aside from socket wiring) between the 7591 and 6L6 is the tube's transconductance. Transconductance (Gm) is described in units of mhos (usually in so many thousands of micromhos), which is the inverse of ohms.

Ohms (from ohm's law) can be written as "Volts/Current". As mhos is the inverse, it can be written as "Current/Volts". Therefore, you can understand this number as "Amperes of plate current"/"Volts applied between grid and cathode". Thousands of micromhos then translates to mA/V; a tube with a Gm of 1200 micromhos can also be described as 1.2mA/V. For every volt applied to the grid, 1.2mA of plate current will flow.

The 7591 has a much greater Gm than the 6L6. What this means for you is that to get the same plate current output (and therefore power), you will need a larger driving signal from the phase inverter/driver. Also, if you use a cathode bias resistor, the 6L6 will need a larger value than the 7591. If you use fixed bias, you will likely need to alter (or adjust) the circuit for a larger negative voltage to arrive at the same idle current from the 6L6 as you got with the 7591.

[Willabe]

I thought that when I said "little bit" and "probably not enough to make a huge difference" that I was just putting that out there for him to be aware. I had no idea I was saying that you were wrong until you pointed it out to me.

I guess I really need to be more reserved and stop carelessly sharing facts.

Dave

FWIW, I don't think you said any thing wrong Dave. What you said was/is true and IMO you didn't step on anybody's toes.

--- IF --- gldtp99 is/was trying make a joke, at best I think it got lost in translation, and --- IF --- at worst, was NOT necessary and NOT funny at all.      

Hope I'm wrong.


                 Brad      

[firemedic]

+1 on keeping the 7591, per the datasheet it is best to keep the 2 screen pins externally connected. Man that is a great sounding tube. The EH new production also sound great compared to the worn out 1965 originals I have.

I use a Deluxe OT (041318), usually used in Fenders for 2-6V6s in P-P. Cathode biased 7591s are a pretty close impedance match with that OT and it sounds WAY better than the stock OT did in the Reverberocket 2. I don't know what OT you have there but if it appears large enough & decent enough quality, I'd use it. The RR2 stock OT was much too small.

Also +1 on the small value cathode resistor, meaning also IIRC that the 7591 requires much less grid voltage swing.

[oldhippy]

Thanks for all the feedback; some food for thought here. The PT is putting out 315V, open circuit. No one seemed to be concerned with any impedance mis-match so I quess I'm close enough? I think the 6v6's would be my tube of choice. I really don't think this OT is big enough to handle 6L6's or the 7591's it came with. I don't have a LOT of experience with tube amps but the OT seems more like what I would expect to see in a deluxe or something 6V6's. This amp came with a 20K resistor between the power supply rail node feeding the OT CT and the node feeding the screens. The OT CT read 400v while the screens read 127v. It sounded cold and thin and not nearly as much volume as I expected. It looked original and was true to the schematic on the cab. I tried to upload the schematic a while back so I could get some feedback on it but the file was too large and I don't know how to reduce it so it will post. I'll take another stab at posting it tonite...

[tubenit]

I tried to upload the schematic a while back so I could get some feedback on it but the file was too large and I don't know how to reduce it so it will post.

Try to get the file in a GIF format & then open it with "Paint".   Look for something/anything that says "Resize" and resize it to 45%.

Try posting that after resizing.

With respect, Tubenti 

[Dave]

The 8K ohms and the 6K6 ohms you are referring to are not fixed numbers. Many things can effect load matching. Without begin very familiar with the amp that you are working with it is difficult to answer exactly how something would work out.

You "should" be able to expect that the original manufacturer had some reason for using a specific tranny with a specific output tube.
If the manufacturer did a reasponably good job at balancing the load then a 1k ohm difference in primary impedance might make a noticeable difference.

My suggestion would be to run some tests on the transformer itself and find out what the primary impedance is.

The easiest way to do that, in my opinion is to apply apply a small AC voltage to the primaries (usually the blue and brown wires ignoring the red one) and use a meter to see what comes out of the secondaries. Then divide the voltage you applied to the primaries by what came out the other end. That will give you the turns ratio. Easiest way, use a variac to apply voltage to the primaries and run it up until you see 1 volt on the secondaries. That makes the math easier.

Then then square the turns ration. Then multiply by the number of ohms the speaker is supposed to be. Example 40 X 40 X 8

The resulting number is your primary impedance. Then look at the tube charts and see if the tube you intend to use, under the conditions that you intend to use it would be happy with those numbers.

Dave

[oldhippy]

Thanks once again, Tubenit, it is appreciated. I will give it a try. I would like some qualified feedback, as I have shown this circuit to a couple of tube techs and they say I must have it wrong...but I do not; that's the way it is. Could this have been done to "throttle back" the output for the smaller OT? The whole amp, from the schematic to the cab to the chassis just feels like a one-off, a prototype or a hand-built.
respectfully, S

[oldhippy]

I was just looking at the schematic again and took note of the bias arrangement...seems like a bad way to do it to me; when hitting the standby switch, won't the plate E hit the tubes before the bias cap has a chance to charge to it's working value? thoughts? The schematic I posted was obviously not the original, but was copied as it was glued in the cab. It was ck'd twice against the actual circuit for accuracy. I added the 1 ohm cathode R's, they are not original...Thnx

[HotBluePlates]

Thanks once again, Tubenit, it is appreciated. I will give it a try. I would like some qualified feedback, as I have shown this circuit to a couple of tube techs and they say I must have it wrong...

It looks perfectly reasonable to me. I'm surprised they thought there was some error. Other than the split-load inverter's load resistors being 56k and 50k (I do understand the thought to "balance the resistors in light of the 3k bias resistor).

And you're probably right that the bias will come up a little slower than the B+; moving the standby might be a good choice. Then again, it's worked for how many decades now?

[sluckey]

I'm surprised they thought there was some error.

I think there is an error too. The 250K resistor that feeds the bias diode is shown connected to the cathodes of the B+ diodes. I bet it is really connected to one side of the PT HT winding in the amp.

[HotBluePlates]

You're right Steve! I guess my eyes saw what they wanted to see (bias derived from raw secondary a.c.).

[DummyLoad]

I think there is an error too. The 250K resistor that feeds the bias diode is shown connected to the cathodes of the B+ diodes. I bet it is really connected to one side of the PT HT winding in the amp.

agreed, fixed bias - i see 25K in || with 35K inclusive of the bias pot. the 250K should be connected to the one of anode|25R nodes.

--DL

[PRR]

The bias supply is surely mis-interpreted, as others say.

The power rail 20K and 2K are probably swapped, an easy thing to do in the heat of production. 2K would be a fine screen-dropper, giving a rational 350V. The higher 20K is appropriate for just the small tubes which will draw smaller current and need cleaner power.

This is perhaps THE reason the amp was unhappy. 127V on screens is just so-so-wrong. The poor starved 7591s won't make 10 watts this way. The low screen voltage with nominal G1 voltage means the outputs idle cold. The low B+ to the driver means it can't, even when provoked, swing 7691 G1s to get even the small power the low screen voltage allows.

Swap these two parts and see if it wakes up.

The tone control seems to be the James but mis-drawn or mis-built.

The cathodyne "ought" to either have 56K in bottom or have lower output taken at top of the 3K cathode resistor which then would be 5.6K (but that might mis-bias the tube). I really do not think this 56K:50K "unbalance" matters to the ear; leave it.

> I really don't think this OT is big enough to handle 6L6's

It won't melt. It may have strong color on your bottom notes; this can be useful, or you can shave some coupling caps for a lighter tone.

Real 7591 are all gone and new-made "7591" are not stocked everywhere, may be of variable quality. I think the 7591 was used because, at the time, it was cheap; things change and I'd re-tube it.

400V into 6.6K is heavy abuse for 6V6, although some amps do it.

I would install 6L6. The heater current difference, bah. You DO need to change the bias significantly. You could run 6L6 self-bias at 24 watt/each dissipation. Personally I'd remove the 25K drawn near output cathodes; that may be enough to up the bias to -40V suitable for 6L6.

[HotBluePlates]

The tone control seems to be the James but mis-drawn or mis-built.

I saw it as James tone circuit + volume control at output.

[oldhippy]

Lots to think about...thanks. And thanks for the input on the screen voltages, PRR. I wished I hadn't already stripped it out or I would have switched the R's as you suggested...I got impatient. I wish I had taken a gut picture of it, it was a mess.Would there ever be a legitimate reason to run a screen E so low? Curious...like I said, that's the way the cab schematic had it shown.
I'm going to preceed with the small-ish looking OT, since you didn't seem too concerned about it. Since I've got the time and a new-ish  pair of EH 7591's that otherwise would probably sit here forever,  I'll use them at least for the time being in this TOS build, cathode biased, So I will most certainly have more questions at some point along the way. respectfully, S