Using 6L6's in a 6V6 circuit

[John Fisher]

Greetings! It has been a while since I have been here. I have at times got into tube amps and I swing between that and buiding guitars so I am a little rusty at this point.
The problem:

I have a Deluxe 5D3 that I built that I really like that I play at gigs.
I am living in the boonies here in Mexico and I need some new 6V6 tubes for the next gig in a couple of days. They are not redily available where I am so I have to wait. I do have some 6L6's and I was wondering if I can just stick some of those in to replace the 6V6's temporarily. Will that overload the OT? I also have a couple of 5U3 recifiers to replace the 5Y3 if necessary. is all this safe or will it in some way overload the OT?

Thanks!
John Fisher

[hasserl]

Hey John, good to hear from you again. Hope all is well. I think the biggest danger you have in using 6L6's in place of 6V6's is with the heater current draw, with 6L6's drawing about twice what 6V6's do. The danger is the added current draw could cause the power transformer to over heat and fail. With the OT the reflected load from the speaker will be about twice what it should be for 6L6's, but I don't know that it will cause any damage, it's possible but not inevitable.

[tubeswell]

What Hasse said. If your PT has an Heater winding of about 3A min and 180mA on the High Tension winding, you should be alright. If it has a little old stock Fender 5E3 PT, you will screw your PT. (IIRC the stock 5E3 PT has a 2A heater winding and a 70mA High tension winding)

If on the other hand, it can take it, then pop a GZ34 rectifier in as well and get an extra 50V or so B+ for running the 6L6s. But you probably also want to run a speaker in there that is rated at 1/2 of what the nominal speaker load is (i.e. 4R if the usual load is 8R, or 8R if the usual load is 16R, and so on).

[John Fisher]

Well I know that the PT will handle because of it's rating but I was more curious about the OT (Home made) and also the recitfier tube. Will it be ok to put in an 5U4 instead of a 5Y3? Will all this give me more power similar to the old 25 watt Fender Super amps or will it remaim the same at about 15 watts? I guess I was kind of thinking it was like in a Solid state circuit where even if you put in bigger transistors it will still run at a certain current depending on the components around the transistor.
Thanks!
John Fisher

[tubeswell]

A 5U4G needs a 3A rating on the 5V winding. It will increase the B+ by about 30V

[John Fisher]

I know the power supply can handle all this as I had it made a while back for a 40 watt amp and it can handle the heater current and the rectifier current easily. I think my main concern is that I don't want to fry the OT. Is that a valid concern? Would it be wise to maybe increase the value of the cathode resistor? I am not too concerned about a lot of volume. The amp is already pretty much too loud as it is for what I do as I play a lot in resturants. I don't want to give the poor folks heart burn. Ha!
I did try it and it sounds really good with more head room. I like it!
Thanks!

[txstrat]

Just using 6L6 instead of 6V6 won't fry the OT. Usually a 100% (i.e. 8 ohms to 4 ohms) mismatch is OK. Bias the new tubes to a proper value.
I think there won't be a LOT of more headroom since the 6L6 are ABLE to put out more power but the OT is rated at about 15W. The 6L6 won't change that.

When I think about this I have a figure in mind like this: the current coming from the PT goes through the preamp and power amp being modulated (by the signal) and sent to the speaker. The last device in this chain is the OT which only let pass as much current as it is rated at, to drive the speaker.
I hope this is somewhat understandable.

[John Fisher]

Thanks for your help as I think it's becomeing clearer.
What I am gathering is that because of the higher impedance of the OT that is usually about 8K (primary) and then using the lower impedance of the 6L6 tubes typically about 6K6 it will not force more current to flow through the OT in which it will not cause it to burn up. Correct? That would be why if I was to use a 4 ohm speaker instead of an 8 ohm speaker it will help to rematch the impedance of the 6L6 tubes. ?!

If I was to bias the tubes where the cathode resistor is, what would be the best bias current for the 2 6L6 tubes?

[txstrat]

Using the 6L6 with an OT for 6V6 changes the output impedance. The OT has two windings with a fixed ratio. Seeing 8K of two 6V6 it's 8ohms (10:1). Less primary with the same ratio means less output impedance. That's why you change to a speaker with 4 ohms as tubeswell proposed.

The 250 ohms resistor is a given value of old Fender schematics (I don't know how much current that would be). The higher the value of the cathode resistor, the less current can flow = the tubes run cooler.
I wouldn't go under 250 ohms. 270 ohms or maybe 300 ohms is save (use at least 5W, better 10W with 6L6).

[tubeswell]

Thanks for your help as I think it's becomeing clearer.
What I am gathering is that because of the higher impedance of the OT that is usually about 8K (primary) and then using the lower impedance of the 6L6 tubes typically about 6K6 it will not force more current to flow through the OT in which it will not cause it to burn up. Correct? That would be why if I was to use a 4 ohm speaker instead of an 8 ohm speaker it will help to rematch the impedance of the 6L6 tubes. ?!

If I was to bias the tubes where the cathode resistor is, what would be the best bias current for the 2 6L6 tubes?

yeah the load resistance (which you get from the reflected load through the OT, which is ultimately determined by the speaker impedance) of a 6L6 is fine anywhere between 2k5 - 6k6, but it works better if you can narrow that down to something like between 3k2 and 5k. That's why I said if you were to use a speaker rated at half of what the nominal OT secondary is 'rated' at for a 6V6, you will get a good load resistance for a 6L6.

The cathode resistor is sets the bias (not the load resistance). Using the same value cathode resistor as you would for 2 x 6V6s will give you a good ballpark bias voltage for 2 x 6L6s all other things being what they are. (Assuming you want to run the 6L6s about 30-60V higher on the plate than you would for the 6V6s, which most likely means chucking in a higher-rated rectifier). But you may want to replace a 5W cathode resistor with a 10W cathode resistor of the same value. YMMV