List of compatible tubes/12ax7/6l6/kt88/el34???

[beuoy]

Where can I find a list of what tubes I can use. I see sites like EuroTubes that use kt88 and 6l6GC at the same time with no mods in a 5150. Dose this work? If you answer no do you really know why or are just repeating what others have said.?

I have a Block letter 5150, '66 Super Reverb, and a SF Bassman 50 that I would like to experiment with but I would like some idea of how far I can go.

Is a bias setting all I need to go from 6l6 to el34 to kt88. Or even kt66, kt77, 6v6.

I have read quite a bit but I can't find any solid info on what works and what dose not. This has nothing to do with personal taste just what will works and what dose not.

[telemoxy]

Where can I find a list of what tubes I can use. I see sites like EuroTubes that use kt88 and 6l6GC at the same time with no mods in a 5150. Dose this work? If you answer no do you really know why or are just repeating what others have said.?

I have a Block letter 5150, '66 Super Reverb, and a SF Bassman 50 that I would like to experiment with but I would like some idea of how far I can go.

Is a bias setting all I need to go from 6l6 to el34 to kt88. Or even kt66, kt77, 6v6.

I have read quite a bit but I can't find any solid info on what works and what dose not. This has nothing to do with personal taste just what will works and what dose not.

I would direct your questions to Eurotubes if it is their idea.

It is not advisable to change power tubes without checking and adjusting the bias so I always perform this step.

I have my Ampeg VT40 wired to accept 6L6GCs, EL34s, KT88s and 6550 but I did a lot of research, a very knowledgeable friend helped me do it and the VT40 has the ability to handled the voltages involved.

You might try talking to a competent tech in your area and see what he says.

[tubeswell]

Okay I'll have a go a answering this.

Things to consider in swapping tubes

Pre-amp tubes generally in most amps the 12A-7 family are pretty much interchangeable if they are in a stock fender type circuit where they see between 150-250V on the plates and have a cathode bias of around 1.5-2.5V. They generally draw no more than 0.3A heater filament current each. Plate current draw varies a bit - generally draw no more than about 10mA plate current for a 12AT7, and less for most other types, so that aspect is fairly miniscule, and there is usually enough play in PT B+ windings to cope with pre-amp tube substitutions. If in doubt, consult the relevant tube datasheet. You can for example pretty readily swap a 12AX7 for a 5751 or a 12AT7 no problems, and all you will notice is a drop in gain. Similarly you can sub a 12AY7 for a 12AT7. But if you try and sub a 12AX7 for a 12AU7 or 12AY7, you might have to do some gain stage circuit tweeks in order to not end up with excessive voltage or unpleasant chirps and odd noises (scatchy pots etc) in the subsequent signal path through to the following stages.

Output tubes are not quite as simple, and there are a range of ways of setting them up (cathode bias, fixed bias being the two main methods of biasing - There is grid biasing, but I won't go there today, and probably neither should anyone else these days I gather). For cathode biasing you have to be careful to ensure you have the appropriate value of cathode resistor(s) to get an adequate level of relative negative bias on the grid. Similarly for fixed bias, you have to ensure the idle grid voltage is sufficiently negative w.r.t. the cathode to prevent red-plating. All this can be different for different tube types, and is also dependent on the plate and screen voltages, as well as load resistance. The tubes have to work against a load to use up all that energy in a pleasant way, so you have to make sure the combination of tubes you are putting in has the right ballpark of load resistance through the OT and speaker(s). Then there is the heater circuit to consider - output tubes generally draw more heater current than pre-amp tubes (and this varies e.g. 450mA for a 6V6, 900mA for a 6L6). So the heater winding on your PT has to be meaty enough to take the total current draw of all the tubes your are putting in, otherwise your PT may catch fire or explode. Similarly for the PT's HT/B+ winding, you have to make sure that the voltage it puts out, in combination with the rectifier and filtering you are using, is appropriate to drive the tubes, and can handle the combined maximum current draw from the plates and screens. You also have to figure-in the load resistance presented by the speaker(s) through the OT, and whether it is sufficient/appropriate to load the output tubes enough to get a pleasant sound without making your output tubes implode or red-plate. If, for example you want to substitute a 6L6 for a 6V6, you have to make sure that the load resistance is appropriate for the plate voltage you are going to get through the OT, which will depend on the plate voltage, and current, which will depend on the method of biasing (fixed or cathode) that you want to get you into the Class of operation (A, AB1, AB2, B) that the output stage is set up for so that you don't end up with unpleasant noises or worse. Ah, I've run out of energy for now (maybe somebody else can continue)... I didn't talk about what to watch out for in rectifier tube substitutions

[beuoy]

So, from what I see here as long as I know how to setup the amp to accept the tubes I put in it all is well? Worst case is I might need a fire extinguisher? I just hope it happens when there is a crowd to witness it.

I know most players are scared to even turn the knobs up. Scared to death they might blow something up. This is not me. I doubt I would hack the hell out of my '66 SuperReverb but the 5150 is fair game. There isn't much there
that can't be fixed (this goes for the SR also) even if the OT goes south and takes with it whatever. Believe me, I don't like blowing stuff up but every time it happens I learn something and that is worth more than the fix it requires. OK so I haven't blown up much (I've been lucky) but I'm willing to take a chance if it means a better end result.

What I really would like to find is a tube compatibility chart or something along that line to give me a place to start.

[tubeswell]

Well I hope I didn't mislead you - its not so much as simple as setting it up right in terms of where output tubes are concerned - you might have to put in a different PT or OT, depending on what you are substituting. That may mean overhauling the whole amp, and it might be just as easy to build a different amp. But then again, your amp might have what it takes already. You probably should at least find out what the ratings are on the PT and OT first

[beuoy]

man you stay up late!!

Thanks for your help. I think I will wait and look up the PT/OT specs tomorrow.

[Steve Conner]

The main things to watch out for:

EL34, KT88, 6550 all draw more heater current than 6L6GC and may burn a PT designed for 6L6, or worse, 6V6. They're also physically bigger and may not fit in an amp designed around the 6L6GC. (For instance, I heard KT88s won't fit in a 5150.)

EL34 needs a different bias voltage and can't tolerate so much screen voltage as the other tubes. 6550 is halfway between a 6L6 and an EL34 in this respect.

6V6 is a small low-powered tube and may die if substituted into a high-powered amp. You can buy super heavy-duty 6V6s nowadays, though.

If you don't understand terms like heater current, bias voltage, and screen voltage, you may want to get help from an amp tech.

[tubeswell]

When you hit mid-life you need less sleep than you used to. My Old Dad is still alive in his mid 70s and reckons that if he can stay awake twice as long and do twice as much, he'll be able to stretch out his remaining span of years by four times as much as he would do if he took it easy. On this basis he is looking forward to retirement at 80-something. So taking a page out of his book... ;-)

Besides that, it was only 9:00pm over here when I posted that last (- and now its only 11:00pm)

[stokes]

Another thing to consider when using an EL34 in a Fender designed for 6L6 types is to be careful that nothing is connected to pin 1.Pin 1 should be connected to pin 8 with an EL34,since the two tube elements are not internally connected as they are in a 6L6.In some Fender,and other amps as well,the unused connection at pin 1 on the socket is used to mount other components.

[telemoxy]

The main things to watch out for:

EL34, KT88, 6550 all draw more heater current than 6L6GC and may burn a PT designed for 6L6, or worse, 6V6. They're also physically bigger and may not fit in an amp designed around the 6L6GC. (For instance, I heard KT88s won't fit in a 5150.)

EL34 needs a different bias voltage and can't tolerate so much screen voltage as the other tubes. 6550 is halfway between a 6L6 and an EL34 in this respect.

6V6 is a small low-powered tube and may die if substituted into a high-powered amp. You can buy super heavy-duty 6V6s nowadays, though.

If you don't understand terms like heater current, bias voltage, and screen voltage, you may want to get help from an amp tech.

These are good points and relate directly to the mods I did on the VT40.

The VT40 PT is set up for 7027As (a beefy variation on a 6L6GC) so I had the current draw covered. There is plenty of room in a VT40 for taller power tubes.

We connected pin 1 and pin 8 and reduced the screen voltage to allow the use of EL34s and 6550s. I cannot go back to 7027As but the SED 6L6GCs I substituted for them are a great substitute so it doesn't matter.

If you don't have the knowledge it is advisable to get it or hire it.

[telemoxy]

Okay I'll have a go a answering this.

Ah, I've run out of energy for now (maybe somebody else can continue)... I didn't talk about what to watch out for in rectifier tube substitutions

You captured my curiousity. If your energy is recharged, what is there to watch for in rectifier tube substitutions? I am considering switching to 6L6s from 7591As in my Ampeg Rocket so this discussion is on point.

[tubeswell]

For tubes, same thing as for heater considerations. Its fairly well-known that rectifier tubes have different current draws. e.g.; 5AR4 = 1.9A, 5Y3GT = 2A, 5U4G/5U4GB = 3A, you need to make sure that your 5V recto winding is sufficiently rated current-wise to cover the type of rectifier you are going to use, otherwise you risk smoking your PT. Different rectifiers also have different current (as well as voltage) outputs which you need to consider when deciding how to use them in conjunction with your PT and other-tube configuration.

For diode rectifiers, you need to be aware that silicon diodes have to have a peak inverse voltage 2.8 times the RMS voltage being delivered by the transformer. e.g.; for a 340V -0-340V winding with full-wave diode rectification, you need a rating of 952V on each of the diodes. If you are using 1N4007s with anything much higher than that, you should put two or more in series on each end of the winding to give you extra voltage-handling capability. In order to get the diodes carrying an equal share of the load, you can put a cap in parallel with each diode.