I've been eyeing a pair of old Davoli-Krundaal guitar amplifiers from 1964 (schematic attached). This particular auction is selling a pair, missing 2 out of 4 EL504 and both ELL80 phase splitters.
Since these tubes are rather hard to find nowadays I'm thinking of changing the output section in some way.
These are the ideas I'm pondering, in order of preference:
Either way, I'll probably have to swap out the EL504s and the ELL80s, so what tubes do you recommend as a replacement? As stated I don't really mind changing the sockets/circuitry.
Bonus points if the suggestion includes Russian sweep tubes that can be had cheaply and in abundance (I always like to buy extra for replacements).
Since these tubes are rather hard to find nowadays I'm thinking of changing the output section in some way.
These are the ideas I'm pondering, in order of preference:
- (Hopefully) convert these into a pair of monoblocks for music reproduction. This is the only way I could get some use out of them. My main fear is that the OPTs will have a ugly response outside of the guitar frequency range. Then again, they are rated for 60W so they may behave well at lower wattages (I don't need more than 8W of power).
- Do a proper restoration: try to keep them as stock as possible and only fix what is missing/broken.I may end up replacing just the the ELL80 as it's the hardest tube to find. In the end I'd then have to sell them as I don't play guitar and certainly don't need two of them.
- Change the output section, keeping them guitar amps. Again, I'd have to sell these later.
Either way, I'll probably have to swap out the EL504s and the ELL80s, so what tubes do you recommend as a replacement? As stated I don't really mind changing the sockets/circuitry.
Bonus points if the suggestion includes Russian sweep tubes that can be had cheaply and in abundance (I always like to buy extra for replacements).
Attachments
I am wondering, with all those issues, why you want this particular pair?
Do they have a specific sound? Will they still have that after you get done with them?
Just curious.
Jan
Do they have a specific sound? Will they still have that after you get done with them?
Just curious.
Jan
It would be interesting to understand better why the ELL80 was used as a phase splitter. I don't think there is much problem getting EL504 replacements, and they are cheap too.
Thank you both for the swift replies.
I didn’t mention this previously but the whole scope of the project is really to try new things out on a budget. I’m new to both push-pull amplifiers and pcb design/etching, and I’d like to experiment a bit.
Mainly, they are cheap. With the amount of money I pay a single unit I wouldn’t even be able to get an OT. That’s why I’m totally down to completely changing the topology. I’m essentially buying the iron + a nice (albeit, in need of some repair) housing.
Not matched, yes. But i’d need four matched tubes or two pairs matched, and these would cost me more than the two amplifiers together (searching quickly I’ve found a quartet for 120EUR). That’s also why I’m interested in russian tubes. Cheap, plentiful and easy to get matched.
I didn’t mention this previously but the whole scope of the project is really to try new things out on a budget. I’m new to both push-pull amplifiers and pcb design/etching, and I’d like to experiment a bit.
About the EL504, you can get NOS PL504s for very little money, just a small mod adding a separate 27v heater transformer.
You can use EL36 instead as these are still available. Not sure what other tubes you could use as these have lower Ri then usual output tubes.
Maybe GU50..
The driver could be replaced by a 6360 tube with some minor changes.
Maybe GU50..
The driver could be replaced by a 6360 tube with some minor changes.
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EL509 could be used to, but not cheap these days, even the soviet versions (6P45S) are quite expensive.
Instead of ELL80, I suggest using 2 pieces of EL95 with adapter (2x7-pin MINIATURE socket on a small printed circuit board with 9-pin NOVAL connector) because they can be used without any change in the schematic. A pair of EL95 with an adapter is much cheaper than an ELL80, and it will be easier to replace the tubes afterwards. Instead of EL504, very cheap Russian 6P44S can be used, although EL504 are not too expensive and difficult to obtain. The ECC82 is available, but in this type of amplifier it is also the biggest cause of distortion, so I suggest replacing it with a 6CG6, 6FQ7, or 12BH7 with minimal changes to the cathode resistors (or even without any changes will give better results than those with the ECC82!). Of course, all electrolytic capacitors should be replaced (regardless of what the eventual measurements show to be correct!), and I suggest replacing all foil capacitors as well. In practice, mostly when servicing those (and other Davoli-Krundal amplifiers) I changed the electrolytic capacitors with new ones of higher capacity, mostly double. On the adapter plate for 2 pieces of EL95, I would leave the option of using PL95 tubes, which are cheaper than EL95 and somewhat easier to find. They have a heating voltage of Uf=4.5V with a current of If=0.3A, so for connection to 6.3V, a 3 Ohm resistor with a power of ~2-3W should be connected in series for 2 parallel connected PL95 heaters, on which the voltage is reduced by required 1.8V.
They probably wanted something powerful to the EL504s. A 6N6P would do.
PS Just noticed, EL504 grid leaks are 33K. They did need quite a bit of power from the phase splitter.
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I think this is a bit spurious. You have two amps and need 4 tubes. You should probably get a spare set of tubes. If you buy 10 you should be able to match 2 pairs from that number
EL504 and PL504 exist in large numbers.
From Electools that would just be €100 for 10. From Billingtons you can get 30 NOS Tungsram EL504 for £150; surprisingly PL504 are more expensive, £6.50. PL95 are £2.70.
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Before deciding to turn it into Hi-Fi amp, first thing I would check the actual performance of the output transformer if not specified. Guitar OPTs are not supposed to do 20KHz flat, 10KHz @-1 dB is already more than enough. Generally speaking, they are wound with minimal interleaving and often asymmetric geometry that results in high leakage inductance. It could happen that despite the basic geometry they do well at higher frequency but they won't at low frequency. This means that they have not enough turns to support full power down to 20-30Hz. Guitar amps don't need to go below 80Hz....
One can already guess the power rating by looking at the size.
The ELL80 appears to be triode connected and so has mu=17 like the ECC82. It works at higher current but given the kind of amplifier it's not necessary. Another ECC82 will do the job.
For the output I would look at the basic Russian 6P3S (6L6) which should work fine with those anode and g2 voltages and there is enough range to adjust the bias. Both heaters of ECC82 and 6P3S work at the same 6.3V and consume less current than ELL80 and EL504, respectively.
One can already guess the power rating by looking at the size.
The ELL80 appears to be triode connected and so has mu=17 like the ECC82. It works at higher current but given the kind of amplifier it's not necessary. Another ECC82 will do the job.
For the output I would look at the basic Russian 6P3S (6L6) which should work fine with those anode and g2 voltages and there is enough range to adjust the bias. Both heaters of ECC82 and 6P3S work at the same 6.3V and consume less current than ELL80 and EL504, respectively.
The possible EL504 replacement by PL504's has already been mentioned. Anyway, a renowned German distibutor sells PL504's at € 12.10, but EL504's at € 8.52 - see #12 above. ELL80's aren't available anymore, the also mentioned replacement by a pair of EL95's is a whopping € 33.14! The nude adaptor also isn't available, but one could build it - it's DIY here. Note there's the PLL80 (12.6 V heater) at € 17.90. EL95's are € 12.33, PL95's are € 3.81.
Other possibilities: Philips QQE03/12 (€ 7.05) or Telefunken 6939/QQE02/5 (€ 33.71). Would require socket rewiring, of course.
I'd also try to replace the ELL80 by a double triode, maybe ECC82, as yet suggested, or 12BH7 or something like that.
Best regards!
Other possibilities: Philips QQE03/12 (€ 7.05) or Telefunken 6939/QQE02/5 (€ 33.71). Would require socket rewiring, of course.
I'd also try to replace the ELL80 by a double triode, maybe ECC82, as yet suggested, or 12BH7 or something like that.
Best regards!
The 6P3S tube is basically the same as the 6L6GA and is not suitable in this case as a replacement for the EL504. The output transformer has a primary impedance of Raa=2.5kOhms, which is certainly too small to use 6P3S. In addition to the EL504, the already mentioned 6P44S and 6P36S, EL36, EL360, E235L, E236L, 6DQ6B, as well as a large number of other electronics (mainly designed for horizontal output stages in black and white TVs) would also be suitable here, with a customized connection and/or changing of the base. and correcting the negative bias of G1. This amplifier is not primarily intended as an exclusive amplifier for the electric guitar, but as a universal amplifier and has quite large and well-dimensioned output transformers because the possibility is left to be used eventually for organs and other types of keyboards that have a much wider frequency spectrum than the el. guitars. In practice, musicians also used it for bass guitar, of course with appropriate speakers for that.
Thank you, I didn't know about Electools, great place to buy tubes! This is definitely a possibility. I may also have to change the other pair as I don't know how strong they test. At that point, russian tubes become way cheaper.
Thank you, they this also is a viable option.
All available and cheap, thank you for the suggestions!
Thank you for all the suggestions and the detailed description, yet another great idea for the output tubes! Also the ELL80 to 2xEL95 is a good mod to put my pcb making skills to the test. As I understand it you have some experience with these amplifiers (either this model or other Davoli-Krundals) so do you happen to know if these OPTs are any good outside guitar frequencies?
Since the EL80 are triode connected these are a great swap for the original topology. Thank you!
Yes, that's what I'm worried about too. The trafo is certainly rated for 60w and has an impedance of 2.5k, but that's all I can evince from the schematic. My hope would be that even outside the frequency range it was designed for it would perform decently as I'd drive it at way lower wattages. Do you know if there's a way to find out how it behaves, possibly without the output section working? At first, neither amplifier will be working as I won't have the phase splitter EL80. Even then, caps and resistors will have to be changed all over the place so it will take some time before I can get a working output stage to drive the OPT. Tools at my disposal are: a tiny signal generator, multimeter, an oscilloscope and a benchtop power supply.
Great. I do have some spare on hand so I may use these to see if I can get it to work quickly and do my measurements on the OPT right?
I do especially appreciate suggestions like these involving "modern" tubes. I like having replacements for any failure down the line by either stocking or using still-in-production tubes.
Thank you for the suggestion. I'll look at all these once I decide if to convert the amps.
Great, great news! this gives me hope for an "hifi" conversion.
Thank you so much for the price sum-up! that matches what I've been seeing too. I'm also leaning for swapping the ELL80 with a double triode, maybe one of the 6CG6, 6FQ7, or 12BH7 as @davorin suggested, other than the ECC82.
I think we have enough suggestions for me to pick, really. But please keep ideas for output tubes coming if you have any!
As I would really appreciate being able to convert this into a pair of monoblocks for my system, I guess the first order of business is to get at least one working and prove the quality of the OPTs. There is a unit missing just an ELL80, so I'm thinking of replacing that with an ECC80 and properly re-wiring the connections.
Do you think I can get away without changing any caps (ofc, unless I find something shorted or really out of spec) for the few minutes that will be required for testing?
Of course you can. It's always advisable to change/alter one thing after the other and check after each step. If your capacitors aren't paper ones with degraded insulation properties, they most probably don't need any replcement at all.
Best regards!
Best regards!
Hi
my opinion
Forst of all is better to check the quality of trafo outside the circuit. It is not complicated
Then, the 2k5 ohm is low
Maybe is better to convert it with two pair in triode mode of EL34 as output stage
The input and splitter circut are easy to build with ecc81-ecc82 as exmple
Walter
Thank you all for the replies!
I'd love to do so. Is there any guide available online?