Hi everyone,
Hope you are all well. A friend has asked me to repair his 1962 Bird Golden Eagle guitar amp. This website has all the details including the best drawn schematic for it (https://www.backfromthesixties.co.uk/bird-golden-eagle-2-15). I have managed to get rid of the awful popping sounds it was making (dry solder joint reflowed) and fixed the tremolo (loose wire). I have replaced a number of power resistors whose value were way off the schematic.
However, I'm stuck on the reverb. The original reverb tank is there (identical to the one pictured in the website above) but the phonograph cartridges are completely knackered with no hope of resurrecting). I tried putting in a reverb tank from my home made Fender Deluxe Reverb (https://www.hotroxuk.com/modr-4eb3c1b-reverb-tank-rohs-compliant.html) but this does not work either! I'm thinking this might be something to do with the impedances being totally inappropriate for the driver (V6) and recovery (V1) on the Bird but TBH this is an area I'm not strong on. The configuration of the reverb driver for a Fender deluxe reverb is rather different and also usually uses an ECC81 valve rather than an ECC83 and an EF86 for recovery. Also the deluxe reverb runs at much higher voltages than I am measuring on the Bird.
If I shake or bang the tank it the reverb thunder sound does come through but no even a hint of reverbed guitar comes through when I play. Is the driver too weak for it or is there some other issue? Any thoughts or recommendations as to a more compatible replacement tank if that is the issue?
Thanks in anticipation for your help
Cheers and all the best
Tom
Hope you are all well. A friend has asked me to repair his 1962 Bird Golden Eagle guitar amp. This website has all the details including the best drawn schematic for it (https://www.backfromthesixties.co.uk/bird-golden-eagle-2-15). I have managed to get rid of the awful popping sounds it was making (dry solder joint reflowed) and fixed the tremolo (loose wire). I have replaced a number of power resistors whose value were way off the schematic.
However, I'm stuck on the reverb. The original reverb tank is there (identical to the one pictured in the website above) but the phonograph cartridges are completely knackered with no hope of resurrecting). I tried putting in a reverb tank from my home made Fender Deluxe Reverb (https://www.hotroxuk.com/modr-4eb3c1b-reverb-tank-rohs-compliant.html) but this does not work either! I'm thinking this might be something to do with the impedances being totally inappropriate for the driver (V6) and recovery (V1) on the Bird but TBH this is an area I'm not strong on. The configuration of the reverb driver for a Fender deluxe reverb is rather different and also usually uses an ECC81 valve rather than an ECC83 and an EF86 for recovery. Also the deluxe reverb runs at much higher voltages than I am measuring on the Bird.
If I shake or bang the tank it the reverb thunder sound does come through but no even a hint of reverbed guitar comes through when I play. Is the driver too weak for it or is there some other issue? Any thoughts or recommendations as to a more compatible replacement tank if that is the issue?
Thanks in anticipation for your help
Cheers and all the best
Tom
Fender typically used a step-down transformer, which a 12AX7 can't even think about driving. And a 12AT7 with both sections in parallel to drive it. A JJ ECC823 (12DW7 with sections reversed) might work out, though there will a loss of gain; might be necessary to get the reverb input from after the EF86. The transformer (a tiny output transformer - say 7K to 8 Ohms) would connect from the second stage plate to an earlier B+ tap (pad 6). R20 would come out, change R21 to 470 Ohms,
Before you start replacing things you need to find where the signal dies. Start by putting a dummy speaker load on the amp and feeding in a set guitar level signal.
With VR1 at maximum, check the presence of signal at the input grid of the V6 driver tube (pin 2) and check for the amplified driver signal at the output of the driver V6 pin 6, and at the input to the reverb tank after C14. This will tell you if the signal is 1) getting to the driver, 2) being amplified by the driver, and 3) making it to the input of the reverb tank.
If all that looks ok, start at the output of the reverb tank and make sure the signal is progressing to the grid of the recovery tube, V1 (pin 9). There could be issues with the reverb min/max/off reverb switch or the operation of VR7, or even a bad capacitor (C16 or C17).
This should get you started.
With VR1 at maximum, check the presence of signal at the input grid of the V6 driver tube (pin 2) and check for the amplified driver signal at the output of the driver V6 pin 6, and at the input to the reverb tank after C14. This will tell you if the signal is 1) getting to the driver, 2) being amplified by the driver, and 3) making it to the input of the reverb tank.
If all that looks ok, start at the output of the reverb tank and make sure the signal is progressing to the grid of the recovery tube, V1 (pin 9). There could be issues with the reverb min/max/off reverb switch or the operation of VR7, or even a bad capacitor (C16 or C17).
This should get you started.
New ceramic phono cartridges are available.
Not expensive, could be worth a try to rebuild as near original:
https://www.ebay.co.uk/itm/145788137259
Not expensive, could be worth a try to rebuild as near original:
https://www.ebay.co.uk/itm/145788137259
Good call. I don't actually have a dummy speaker but I think it is time I made one. I did try and trace the signal and there was something going through the driver but seemed to be pretty weak and distorted. It was difficult to tell with the actual speakers connected though so a dummy seems like a good way to proceed here. Thanks again
Interesting option but if I used the EF86 before the reverb input as some kind of driver, would I need to add some kind of recovery stage?
I was thinking of taking the input to the reverb from the plate of V2, instead of the grid. For a small increase in gain, bypass R19.
Didn't realize that this tank has a 600 Ohm input - but a 12AX7 still can't drive that (gain of about 1, clipping at about 1V), 12AU7 triode (lo-mu half of ECC823) and 10K:600 transformer would have a gain of about 3.5, could do 25V or more. I have no idea of typical input / output levels for a magnetic tank, let alone a a piezo one... so some experimenting will be required.
Didn't realize that this tank has a 600 Ohm input - but a 12AX7 still can't drive that (gain of about 1, clipping at about 1V), 12AU7 triode (lo-mu half of ECC823) and 10K:600 transformer would have a gain of about 3.5, could do 25V or more. I have no idea of typical input / output levels for a magnetic tank, let alone a a piezo one... so some experimenting will be required.
It looks like a job for a mosfet source follower. The problem you have with using a different reverb tank as mentioned, is you need a good driver, an ECC82 or 12BH7 say. But you probably haven't got enough spare HT & heater power to run it, hence a mosfet might be the answer.
Looking at the schematic the signal for the reverb comes after a lossy tone control, that's why they used an ECC83 I'd guess for gain, to "boost" the IP signal. They could get away with this by using a non standard reverb tank. Trying to fix/re-build the original tank won't be an easy task.
As others have said this is a job for a sig gen & scope with a dummy load. You can use a 1w 10r resistor in a cup of water as a stop gap but obviously having a cup of water around a valve amp can be problematic. It's do-able though if you take care.
Andy.
Looking at the schematic the signal for the reverb comes after a lossy tone control, that's why they used an ECC83 I'd guess for gain, to "boost" the IP signal. They could get away with this by using a non standard reverb tank. Trying to fix/re-build the original tank won't be an easy task.
As others have said this is a job for a sig gen & scope with a dummy load. You can use a 1w 10r resistor in a cup of water as a stop gap but obviously having a cup of water around a valve amp can be problematic. It's do-able though if you take care.
Andy.
Ah I see what you mean! I've ordered some dummy speaker resistors rated at 50W as it'd be good to properly test the signal through the V6.
I might give this a shot - thanks so much.
Thanks Kay. When you say use 'Fender drive circuitry', do you mean put a 12AT7 in the V6 position but reconfigured so that the grids and plates of the two stages are connected (like the Deluxe Reverb driver) with the plate connected to the new reverb output transformer (10kOhm -> 8ohm) and a B+ feed coming in from the C power supply?
Would the fact that the voltage at C is only around 100-150V be a problem?
I doubt that this offer is a complete cartridge.
To the TO: If your unit were mine, I'd copy the Fender drive circuitry, connect the 12AT7 grids to the plate of V2 via a capacitor and get an output transformer of a primary impedance about 10 kΩ, e. g. from a radio with an EL95 final, to drive a more common 8 Ω input Accutronics/Belton/Hammond/Gibbs reverb tank . The original recovery cicuitry can be left alone.
Best regards!
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Sounds like the driver coil is burned out, but the recovery coil is still OK.
Since you're restoring the amp I wouldn't start rebuilding a completely new driver, especially not a bulky expensive Fender copy. Just buy a new tank: Accutronics 8GB2C1G is drop-in replacement for this type of circuit.
Failing that, replace V6 with an ECC81, change R20 to 10k 1W, bias the driver for about 5mA, and hook up to a type F tank. You can reduce R44 to get a higher supply voltage if necessary.
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Thanks Kay. When you say use 'Fender drive circuitry', do you mean put a 12AT7 in the V6 position but reconfigured so that the grids and plates of the two stages are connected (like the Deluxe Reverb driver) with the plate connected to the new reverb output transformer (10kOhm -> 8ohm) and a B+ feed coming in from the C power supply?
Would the fact that the voltage at C is only around 100-150V be a problem?
Brilliant - I think this might be the easiest option and also the best in terms of retaining as much of the original circuit as possible. I need to check whether the owner is ok to get a new tank for it. I have a feeling that there is also some issue with the driver circuit in addition to the coil probably one or more of the coupling capacitors
Yes, taddy, replace V6 by a 12AT7 and rewire the socket according to the Deluxe driver. Connect the new transformer primary to the squared point 6 in the PSU, i. e. the positive terminal of C29.
Interestingly, I've seen a lousy 12AX7 as a reverb driver as well. It was in an Acoustic guitar amplifier from the early 1980ies, model 165, IIRC. And it also used a transformer, again IIRC.
Best regards!
Interestingly, I've seen a lousy 12AX7 as a reverb driver as well. It was in an Acoustic guitar amplifier from the early 1980ies, model 165, IIRC. And it also used a transformer, again IIRC.
Best regards!
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A few early British guitar amps used reverb tanks based on ceramic/crystal phono cartridges rather than the magnetic/coil type transducers as fitted to the Hammond/Accutronics reverb tanks which were more popular in the US. By all accounts Tom Jennings of Vox fame refused to purchase Hammond tanks or to pay them royalties for use of their magnetic/coil patent, so they developed a royalty free system that used ceramic phono cartridges as the send and recover transducers - I've never heard one but by all accounts while it did work it was a far cry from the sweet sound of a Deluxe Reverb.
Try googling 'early VOX reverb repair' and you might come across a successful repair that doesn't involve too much specialty work - check out the link it gives some info about the early VOX reverb tanks.
http://www.voxshowroom.com/uk/misc/reverb_hood.html
Try googling 'early VOX reverb repair' and you might come across a successful repair that doesn't involve too much specialty work - check out the link it gives some info about the early VOX reverb tanks.
http://www.voxshowroom.com/uk/misc/reverb_hood.html
The link in post 1 describes just such a phono cartridge based reverb tank fitted to the amp the op describes.
Complete with pictures of what a diseased cartridge looks like.
I'd rebuild it with replacement cartridges first just to see how well or not it worked.
The Ebay listings are unclear but after reading a few similar listings I'm pretty sure they are complete cartridges with 'needle'.
http://www.voxshowroom.com/uk/misc/reverb_hood.html
Link shows just such a repair.
It states crystal cartridges are not available but now it seems they are available again.
Complete with pictures of what a diseased cartridge looks like.
I'd rebuild it with replacement cartridges first just to see how well or not it worked.
The Ebay listings are unclear but after reading a few similar listings I'm pretty sure they are complete cartridges with 'needle'.
http://www.voxshowroom.com/uk/misc/reverb_hood.html
Link shows just such a repair.
It states crystal cartridges are not available but now it seems they are available again.
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I would not bother with fragile gram cartridges.
The western world is full of single use smoke detectors with robust ceramic transducers inside them craving to be recycled.
It will take a few experiments to get the mountings right but with luck the electronics will not need to be modified.
I always thought that the home constructed ones used DC bias on the coils instead of the magnets that are attached to the springs on the "posh" ones. I am pretty sure the coils were based on the driver transformers from 1960s battery radios.
The western world is full of single use smoke detectors with robust ceramic transducers inside them craving to be recycled.
It will take a few experiments to get the mountings right but with luck the electronics will not need to be modified.
I always thought that the home constructed ones used DC bias on the coils instead of the magnets that are attached to the springs on the "posh" ones. I am pretty sure the coils were based on the driver transformers from 1960s battery radios.
Update. I finally got the dummy load resistors and tested the reverb circuitry and actually it was all good (signal coming out of the driver and when I feed in a sound into the recovery side it emerges at C18) so the issue was just the tank. I went with the cheapest, if slightly fiddly option, as the owner doesn't want to spend very much on this so I bought the vintage piezo electric phono cartridges as russc suggested (thanks mate) and ... success! (... kind of). So we are getting reverb BUT (1) it is pretty weak and (2) it is prone to feeding back or at least behaving rather erratically - sometimes we get a squeal or more frequently a low loud hum that increases in intensity. I think the weakness may result from the circuit design itself as the driver and voltages involved are weak. The feeding back I dunno but I was wondering whether it might be connected to the VR7 pot and the two caps either side of it C16 and C17. This pot is not a user control as the user has three reverb switches (off, 1 and 2) while VR7 is mounted on the side of the chassis and only accessible by removing the back panel so maybe has been added to allow the reverb to be tuned so you get as much as poss without feedback. The reason I thing something is not quite right is that when you put your hand near it it makes a hum (getting louder the closer you get). Is this normal or some issue with those caps (C16 and C17) or the pot itself? The caps reach up from the circuit board to connect with the pot.