Bendix 6384 Fender Showman Mod

[chaosentertainment]

So, I have a quad of Bendix 6384 tubes, and I was going through my tube stash because I'm going to be selling a large amount of them, and I did a little research and saw a video of an amp built around the 6384, and it sounded very good. I was considering modifying my '64 Showman head to take these tubes.

Then I saw on Tube Depot's website a converter to 6L6.
Now I know the screen voltage rating for those tubes is around 300V, and I also know they draw a median of 1.2A heater current.

So do those adapters somehow handle the screen voltage issue? I emailed David from his website and he said it's plug and play, but I thought it wouldn't hurt to see if anyone else knows what problem gnomes can arise from using these tubes.

Also, does anyone know what the maximum heater current draw is for the power transformer on an AB763? Thank you in advance for your time.

 

[anatech]

Hi chaosentertainment,
Well, each 6L6GC draws 900 mA of current. Your tubes are going to draw 1.2 amperes of current each. That might very well be too much for an instrument transformer. You might, possibly get away with that on a single pair mono tube amp, but not if it was a stereo.

Personally, I would enjoy the 6LGCs in that amp. Start fresh for an amp using 6384s and get a transformer rated for the extra 600mA of current per pair. But then, I really like the sound of Fender amps.

-Chris

 

[JonSnell Electronic]

Using heavier heater current will damage the mains transformer as they are not designed to provide that much current!

 

[chaosentertainment]

Using heavier heater current will damage the mains transformer as they are not designed to provide that much current!

Ok, so the modification would have to be adding a proprietary heater current supply to the power section.

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[chaosentertainment]

Hi chaosentertainment,
Well, each 6L6GC draws 900 mA of current. Your tubes are going to draw 1.2 amperes of current each. That might very well be too much for an instrument transformer. You might, possibly get away with that on a single pair mono tube amp, but not if it was a stereo.

Personally, I would enjoy the 6LGCs in that amp. Start fresh for an amp using 6384s and get a transformer rated for the extra 600mA of current per pair. But then, I really like the sound of Fender amps.

-Chris

At the risk of being circuit-slapped for not being able to check right now, does anyone know of a schematic for a good design for these tubes?

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[jean-paul]

If you add a transformer for heater current then just rewire the tube sockets for 6384. It is an excellent tube. Why use converters ? You won't be going back to 6L6 and 6384 lives very long. If you have more of them it is of course better to keep some spares.

 

[anatech]

Hi jean-paul,
I have one word for you. 7581A. 6L6GC characteristics, 35 watt plate instead of the 30 watt plate of the 6L6GC. The best part? Zero modifications required.

The 6384 is designed for military aircraft use, or anywhere there is a lot of vibration. More than a Fender amp combo puts out. The same plate power dissipation and much higher plate voltage screams for a circuit that would take advantage of those ratings. That tube is wasted in a Fender Showman. The screen voltage incidentally is 325 VDC maximum.

So this tube is not actually a good match for that basic amplifier. You would use higher voltage B+ supply along with that extra current for the heaters. It doesn't make sense to use it in your Fender 'cause it will sound like ... a Fender. Build something fresh and come out with an awesome amplifier, and a Fender amp worth it's book value. Modifying the Fender will reduce it's resale value as well.

-Chris

 

[jean-paul]

If the owner wants to use 6384 in his Fender then let him. Not everyone cares about resale value. Maybe he likes the sound better than it was. The guitar and bass guitar players I know just want "that" sound, whatever their amplifier is. They even tube roll with non equivalent but pin compatible types just to find "that" sound. OP also HAS the 6384 tubes, not 7581A. Using an overrated part is OK. Just rewire sockets and make sure a transformer is used that can feed the heaters. I once found a stash of 1952 JAN 6L6GAY here, all NOB/NOS. They almost all ended up in guitar amps as misuse did not kill them. Pity ? Might be, it would even be more pity if they were left unused Still have them NIB.

6384 can resist 500G. It was designed for cruise missiles and about everything in this tube is sturdy. They cost as much as a car when they were produced.

 

[chaosentertainment]

anatech has a point here. It would be a fun accomplishment to butcher this amp to make it work with the tubes, but the key word here is butcher. I dont know if I have the heart to do it. I have a crap load of tubes, and it's funny because I recently sold a quad of 7581A G.E. on ebay, brand new in box. The buyer opened a case behind my back in paypal, and although I responded it was too late they awarded him his money back without having to even return them. I was considering rolling them through the Showman or my 6505+, but I thought to myself I need the funds right now and I can always find more later. True, but at around $450 a quad and only on ebay.

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[jean-paul]

It was your own question and thread title is "Bendix 6384 Fender Showman Mod" (I would have called it "Fender Showman Bendix 6384 Mod" ) There is a thin line between butchering and modifying. Butchering is what is often done in general with guitar amplifiers. Terrible job to repair a butchered one. Wrong parts /wrong tubes/terribly dirty/ haven't had maintenance in 30 years etc.

It is not clever to have better/higher rated pin compatible tubes and sell them (for 0$ even). The Paypal dispute probably went different. For instance sending the tubes weeks after payment etc. IMO Paypal nowadays is quite careful and they ask both sides what the situation is.

* BTW I have a pair of balanced output transformers for 6L6/6384 that I will advertise in Swap meet. Building a new 6384 amp saves you from (God forbid) butchering the Fender.

 

[anatech]

Hi chaosentertainment,
Try using the Electroharmonix 6L6EH. I have in Fender amplifiers like yours and they really last. The plates are the same as some 7581A tubes that I have. I can make that recommendation with 100% confidence that they will last and sound the same as the 6L6GC tubes they are replacing. These are far less expensive than your current options seem to be. Just for interest, the common name for a 7581A is a KT-66 on your side of the pond.

Hi jean-paul,
As a technician, it's my job (and yours) to point out possible issues that may crop up from a planned action. You are exactly right that this is chaosentertainment's choice. But it's only a choice when he has thought about all the factors that come into play here. I'm not making up his mind, just bringing up other things to consider. Something that you did not do. Also consider that the only place for another transformer is on the side of the cabinet. It could easily tear off at some point in time because the wood isn't the strongest material to mount transformers to. I'm going to assume that the amplifier travels. Mounting the transformer over the signal level areas would probably couple the hum right into those low level signals, so you can't do that easily. Then you have those long leads to tend with. Is modifying that Fender amp would properly involve replacing the existing transformer with the higher rated one. Now you are getting into real $$, but this would be the only reliable way to deal with the heavier heater current (+ 1.2 amperes with no safety margin).

-Chris

 

[Tubelab_com]

Using heavier heater current will damage the mains transformer

A lesson that I learned at a young age. Stuffing a 6L6GC into a 50's vintage Fender Champ will eventually fry the power transformer. It took about a year, but it started smelling funny after several months. Then it started blowing fuses, so what does a 10 year old kid do, stuff in a bigger fuse.....smoke cloud.

I'm sure that doubling the heater draw contributed, but I now know that the plate current probably went up too.

I had to rebuild a Bandmaster maybe 25 years ago because someone tried to make 6384's work in it. The Ebay seller advertised that the 6384 was absolutely the best tube for these old amps, and they would never die. He neglected to state that the pinout and bias points were different. When I got the amp the 6384's were rather toasty looking (they are still around here somewhere) and one side of the output transformer was open. The power transformer had an over stressed kind of smell, but was still alive. I rewired the amp for some 6L6GC's and all was well.

 

[chaosentertainment]

It was your own question and thread title is "Bendix 6384 Fender Showman Mod" (I would have called it "Fender Showman Bendix 6384 Mod" ) There is a thin line between butchering and modifying. Butchering is what is often done in general with guitar amplifiers. Terrible job to repair a butchered one. Wrong parts /wrong tubes/terribly dirty/ haven't had maintenance in 30 years etc.

It is not clever to have better/higher rated pin compatible tubes and sell them (for 0$ even). The Paypal dispute probably went different. For instance sending the tubes weeks after payment etc. IMO Paypal nowadays is quite careful and they ask both sides what the situation is.

* BTW I have a pair of balanced output transformers for 6L6/6384 that I will advertise in Swap meet. Building a new 6384 amp saves you from (God forbid) butchering the Fender.

Actually most users open a dispute on eBay where they purchased the item. This buyer opened it on Paypal and circumvented eBay. My paypal email is one that I no longer use for business other than accepting the payment, it's an old AOL account, and I do not check it as often as I should, so I got to it too late and they closed the case. The buyer never contacted me, and offered no reason for the dispute. My multiple messages to him went unanswered.

So how differently do you suppose it went? I have 684 successful sales over 10 years, and that was the only time I ever had a dispute.

 

[jean-paul]

He neglected to state that the pinout and bias points were different.

I thought hotrodding was supposed to be fun ?

Seriously, using non pin compatible tubes without rewiring sockets is just plain stupid. Of course the transformer must be changed for a heavier one when tubes are used that draw more heater and plate current.

I stopped repairing tube guitar/bass amplifiers but IMO the modded ones were often taken good care of as the owner had had high costs for repair before. It were the amplifiers of those that were very fanatic with it that were in the worst shape. Even ultra worse, when it had new tubes and was fully up to spec again they always were complaining it sounded different What struck me was that most of them wanted tube amplifiers but the knowledge of tubes was nil to nothing. Rough handling the amplifier themselves but worried when you cleaned the case with some mild soap and a moist cloth ...

 

[chaosentertainment]

A lesson that I learned at a young age. Stuffing a 6L6GC into a 50's vintage Fender Champ will eventually fry the power transformer. It took about a year, but it started smelling funny after several months. Then it started blowing fuses, so what does a 10 year old kid do, stuff in a bigger fuse.....smoke cloud.

I'm sure that doubling the heater draw contributed, but I now know that the plate current probably went up too.

I had to rebuild a Bandmaster maybe 25 years ago because someone tried to make 6384's work in it. The Ebay seller advertised that the 6384 was absolutely the best tube for these old amps, and they would never die. He neglected to state that the pinout and bias points were different. When I got the amp the 6384's were rather toasty looking (they are still around here somewhere) and one side of the output transformer was open. The power transformer had an over stressed kind of smell, but was still alive. I rewired the amp for some 6L6GC's and all was well.

That is a great story. My buddy and I did the same thing in high school to a '65 Champ...we also built some nifty overdrive boxes and a monstrous universal power supply box called "The Piper", because we paid the piper in electrocutions while designing it. Before I came here I read the data chart for those tubes, so I know there are some significant differences. I was at the very least feeling suspect about the tube seller's claim that his adapters were plug and play. It just didn't make sense when you look at the data.

 

[chaosentertainment]

Hi chaosentertainment,
Try using the Electroharmonix 6L6EH. I have in Fender amplifiers like yours and they really last. The plates are the same as some 7581A tubes that I have. I can make that recommendation with 100% confidence that they will last and sound the same as the 6L6GC tubes they are replacing. These are far less expensive than your current options seem to be. Just for interest, the common name for a 7581A is a KT-66 on your side of the pond.

Hi jean-paul,
As a technician, it's my job (and yours) to point out possible issues that may crop up from a planned action. You are exactly right that this is chaosentertainment's choice. But it's only a choice when he has thought about all the factors that come into play here. I'm not making up his mind, just bringing up other things to consider. Something that you did not do. Also consider that the only place for another transformer is on the side of the cabinet. It could easily tear off at some point in time because the wood isn't the strongest material to mount transformers to. I'm going to assume that the amplifier travels. Mounting the transformer over the signal level areas would probably couple the hum right into those low level signals, so you can't do that easily. Then you have those long leads to tend with. Is modifying that Fender amp would properly involve replacing the existing transformer with the higher rated one. Now you are getting into real $$, but this would be the only reliable way to deal with the heavier heater current (+ 1.2 amperes with no safety margin).

-Chris

All valid points. I don't want to start a ******* contest here, because you both make sense. The amp is a studio fixture. I have plenty of time to think about what to do. But it would seem that the most sense would lie in designing a new amplifier around the Bendix tubes like you initially suggested. I'm going to try those 6L6EH tubes out. It currently had a set of standard EH 6L6GC. Do you know of any current tried and tested 6384 based schematics? I have pair of old EVM 10M speakers, that I have been dying to load up in a 2x10 combo. This might be the vehicle I've been waiting for.

 

[jean-paul]

For HiFi or guitar ? I posted one for HiFi a few posts back. Single ended though.

 

[w5jag]

I would respect the screen voltage rating for 6384. Pin out aside, I would not consider it to be a plug and play substitute for any 6L6 type. I would consider it to be similar to a sweep tube.

6AR6/6098 is what I would use to test with.

Win W5JAG

 

[chaosentertainment]

For HiFi or guitar ? I posted one for HiFi a few posts back. Single ended though.

Guitar, I should have specified.

 

[anatech]

Hi chaosentertainment,
They are the standard 6L6EH. What do you think of them?

As an aside, I have some 7591EH tubes. The plates are very much larger than the original 7591A tubes. The characteristics look to be the same on my tube tester. Once this amp is restored, I'll have an opinion on those too. Also, the 12AX7EH are very quiet. Try them if you haven't yet.

-Chris