New old Thunderbolt: speaker connector and assorted other questions

[passaloutre]

I finally got my Thunderbolt, a 1966 diode-rectified model. It is in great cosmetic shape and reasonably well cared-for electronically. It has all new electrolytics, mostly original signal caps and resistors, and the original output transformer. The power transformer has been "upgraded" to a Mercury magnetics model, which, judging by the several holes drilled in the chassis, is not the first replacement power transformer this amp has seen. In any case, the power transformer seems to work fine, the installation was proficient, and the amp sounds great with its original Jensen speaker.

I did have a few questions regarding this amp, and I know they're somewhat common here so hopefully someone can help out.

The original speaker connector was some sort of two-pin thing (I've seen it referred to as a "DIN 41529"). Someone has left the female side of the connector installed in the amp, but apparently lost the male part, as the speaker wires are just soldered straight to the connector. I know it makes no functional difference, but it would make me happy to reunite the amp with its proper male counterpart. Can anyone confirm the name of this connector and possibly provide a source?

This is the first amp I've come across with no screen resistors. I will mostly be playing it loud. Is it worth installing some for safety, or is this a case of "the amp has been perfectly fine without for 60 years, leave it alone"? Is it going to eat tubes?

Someone has installed a cathode bypass cap on the power tubes. This does not appear in the original schematics, but is nearly ubiquitous in the cathode biased amps that I've worked on. Does anyone have experience with this circuit with/without the bypass cap?

Somebody has previously modified it to cascade the unused triode into the original first triode using the switching jack arrangement in the attached schematic (also reference in this topic: https://el34world.com/Forum/index.php?topic=26084.0). I like both available sounds and intend to leave it as is. My question is this: is it possible to use an A/B footswitch attached to both inputs and be able to switch between the two inputs on the fly? My intuition is that having a cable plugged into that switching jack will break the connection and not allow the signal to pass from the boost stage. Is there another input jack arrangement that would allow this?

The original output transformer is pretty ugly. My experience with 60s era uncovered output transformers is that they always look like something out of a horror movie, but this one looks somehow worse. It seems to work fine, but something appears to have melted or seeped out. Do you see anything concerning in the attached photo?

Some discussions I can find on the internet reference a ground wire that is connected between the amp chassis and the shielding plate inside the cabinet. Mine has no such wire, but seems reasonably quiet and not hummy. Should I install this, and if so, where would it have made these connections in the original amp?

Thanks in advance for your help. I'm sure to have more questions as I get deeper into this thing.

[bmccowan]

I finally got my Thunderbolt, a 1966 diode-rectified model. It is in great cosmetic shape and reasonably well cared-for electronically. It has all new electrolytics, mostly original signal caps and resistors, and the original output transformer. The power transformer has been "upgraded" to a Mercury magnetics model, which, judging by the several holes drilled in the chassis, is not the first replacement power transformer this amp has seen. In any case, the power transformer seems to work fine, the installation was proficient, and the amp sounds great with its original Jensen speaker.

I did have a few questions regarding this amp, and I know they're somewhat common here so hopefully someone can help out.

The original speaker connector was some sort of two-pin thing (I've seen it referred to as a "DIN 41529"). Someone has left the female side of the connector installed in the amp, but apparently lost the male part, as the speaker wires are just soldered straight to the connector. I know it makes no functional difference, but it would make me happy to reunite the amp with its proper male counterpart. Can anyone confirm the name of this connector and possibly provide a source?

- quite a few Valco amps use that connector, but I do not know if that is the correct number. If you can't find it, you could replace the jack with a
 switchcraft phono jack and use a standard plug.

This is the first amp I've come across with no screen resistors. I will mostly be playing it loud. Is it worth installing some for safety, or is this a case of "the amp has been perfectly fine without for 60 years, leave it alone"? Is it going to eat tubes?

- If not being a geek about originality, I would install them. BTW - you say multiple PTs have been replaced? that's not "fine for 60 yrs," eh?

Someone has installed a cathode bypass cap on the power tubes. This does not appear in the original schematics, but is nearly ubiquitous in the cathode biased amps that I've worked on. Does anyone have experience with this circuit with/without the bypass cap?

- I added one to my Thunderbolt - liked it better without and removed it. The difference is not huge - follow your ears.

Somebody has previously modified it to cascade the unused triode into the original first triode using the switching jack arrangement in the attached schematic (also reference in this topic: https://el34world.com/Forum/index.php?topic=26084.0). I like both available sounds and intend to leave it as is. My question is this: is it possible to use an A/B footswitch attached to both inputs and be able to switch between the two inputs on the fly? My intuition is that having a cable plugged into that switching jack will break the connection and not allow the signal to pass from the boost stage. Is there another input jack arrangement that would allow this?

 - Others here have played with channel switching much more than me - so I will resist sending you in a wrong direction.

The original output transformer is pretty ugly. My experience with 60s era uncovered output transformers is that they always look like something out of a horror movie, but this one looks somehow worse. It seems to work fine, but something appears to have melted or seeped out. Do you see anything concerning in the attached photo?

 - Unless its actively seeping now - I'd just leave it. It is an OT that is prone to failure IMO. I've had 2 of 3 fail in TBolts and Harmony 420 - same OT.

Some discussions I can find on the internet reference a ground wire that is connected between the amp chassis and the shielding plate inside the cabinet. Mine has no such wire, but seems reasonably quiet and not hummy. Should I install this, and if so, where would it have made these connections in the original amp?

- In mine, the connection to the shielding plate is right in the center - but neither the connection point location at the shield or the chassis is important. It might reduce EMI. You have a properly installed 3 wire power cord, right?

Edit - I think this is the speaker plug you seek. If it isn't you will need to buy a Hammond B3. https://www.b3guys.com/product/2-Pin-Plug-for-Leslie-Speaker

EDIT: de-tangled the inline quotes --- HBP - thanks

[passaloutre]

Thanks! Do you have a photo of the connector from an actual Thunderbolt? I know a simple 1/4” jacks would be fine, but I like the quirkiness of the original.

I’ll probably go ahead with the screen resistors. Think 100 ohms is enough?

I’m hoping I can figure out the “channel” switching, maybe thinking outside the box and using some special jacks. I’ve seen some pretty brilliant things done with electronics and simple switching arrangements. Switchcraft makes several switching jacks beyond the “shunt” type we normally see. Just browsing Antique Electronic Supply I’ve found some that have isolated make or break switches independent of the signal circuit, and one that’ll transfer the tip connection, like another throw on the typical shunt jack.

https://www.tubesandmore.com/products/14-jack-switchcraft-mono-isolated-make-circuit

https://www.tubesandmore.com/products/14-jack-switchcraft-mono-isolated-break-circuit

https://www.tubesandmore.com/products/14-jack-switchcraft-mono-transfer-circuit

[bmccowan]

Thanks! Do you have a photo of the connector from an actual Thunderbolt? I know a simple 1/4” jacks would be fine, but I like the quirkiness of the original.

I'm not home - but might be tomorrow and will snap a pic

I’ll probably go ahead with the screen resistors. Think 100 ohms is enough?
If you want to read a lot about this: https://el34world.com/Forum/index.php?topic=21489.0 I put 470R 1 watt in my 6L6 amps, but only because I am copying smart people. You said you play loud, 100R might be a bit weak.

Sluckey may be your guy on the channel switching thing

[passaloutre]

Thanks, I would appreciate the photo. No rush.

The 100 ohm was based on some topics I found here and elsewhere specifically related to the Tbolt. I certainly has no complaints about my other 6L6 amps with 470 ohm screen resistors though.

I’ll have to find a good way to mount them, as this amp is already using pin 6 (my usual tie point for the screen resistors) for the grid stoppers.

Since you asked, and I didn’t answer: yes it has a three prong cord, but still maintains the polarity switching. I may rewire that to a more modern arrangement.

[passaloutre]

For the record, I removed the cathode bypass cap and added screen resistors, 100 ohm because I had them on hand. I’ll order some 470s.

With 412v on the plates, 27v on the cathodes, and a 200ohm cathode resistor, I’m at something like 25 watt plate dissipation, so that all seems fine.

I ran some tests with a dummy load and oscilloscope to find the output power. I input a 500mV 500Hz signal to the “low” input and turned it up until just before appreciable clipping, and I had 12.3 volts across the 8ohm load, so about 19 watts of “clean” power. With the same signal into the “high” input and the volume knob dimed, I got a 17.5 volt square wave, or about 38 watts (photos attached).

[bmccowan]

Thanks, I would appreciate the photo. No rush.

The 100 ohm was based on some topics I found here and elsewhere specifically related to the Tbolt. I certainly has no complaints about my other 6L6 amps with 470 ohm screen resistors though.

I’ll have to find a good way to mount them, as this amp is already using pin 6 (my usual tie point for the screen resistors) for the grid stoppers.

Since you asked, and I didn’t answer: yes it has a three prong cord, but still maintains the polarity switching. I may rewire that to a more modern arrangement.

Attached is a pic from one of my amps and this site has good pics showing the cable https://trcrandall.com/products/1961-thunderbolt-s6420
The cable is a Switchcraft product - one pin slightly larger diameter than the other.
You are using pin 6 on the 6L6s, but still have pin 1 available, eh? If not you could use one of the single threaded turrets that are all over ebay: various configurations and prices
And yes, you would be wise to wire up the 3 wire power cord fuse>switch>PT. I think I wired mine so that either of the on positions sent the hot wire to the PT - its been a while - both since I worked on it and played through it. I should move it along.

[bmccowan]

And here is another source for what appears to be the same spec. Rhodes piano and Leslie speakers used plugs that at least look similar to the Valco cable.
https://www.cvkeyboards.com/products/fender-rhodes-and-leslie-speaker-connectors

[AlNewman]

For the record, I removed the cathode bypass cap and added screen resistors, 100 ohm because I had them on hand. I’ll order some 470s.

With 412v on the plates, 27v on the cathodes, and a 200ohm cathode resistor, I’m at something like 25 watt plate dissipation, so that all seems fine.

I ran some tests with a dummy load and oscilloscope to find the output power. I input a 500mV 500Hz signal to the “low” input and turned it up until just before appreciable clipping, and I had 12.3 volts across the 8ohm load, so about 19 watts of “clean” power. With the same signal into the “high” input and the volume knob dimed, I got a 17.5 volt square wave, or about 38 watts (photos attached).

I think you may have to multiply your results by 0.7 to get RMS output at the speaker.  Or measure voltages with a RMS multimeter to get your calculations.  I may be wrong though, I'm still learning as well.

[passaloutre]

You may be right, I tried to find out if my meter was peak or RMS, but it wasn’t specified in the manual, which probably implies it’s not RMS (I imagine they would advertise that heavily). It’s a consumer grade auto ranging Klein from Home Depot, so nothing fancy. In any case, peak to peak jibes with the numbers on the scope, so I’ll go with that. Would the 0.7 be applied to the voltage or the wattage? I’m also still learning.

After some experimentation, I found that the original speaker jack fits reasonably well with a banana plug in the big hole and an rca plug in the small hole so I made up this connector (attached).

Also yes, I used pin 1 to mount the screen resistors, and the power switch is wired so both positions offer the correct polarity.

Thank everyone for the help!

[acheld]

For RMS voltage, it's .71 x Vpeak.

So, for example, if you have a 2 VAC peak-to-peak signal (what you see on your scope), the Vpeak is 1VAC, and the Vrms is 0.71Vrms.

[bmccowan]

After some experimentation, I found that the original speaker jack fits reasonably well with a banana plug in the big hole and an rca plug in the small hole so I made up this connector (attached).

Nice hack - enjoy the amp. Like most of my amps, its too damn loud for what I do now. A friend of mine is in a garage band named Dad To The Bone. They really are all dads and really do play in the garage. I think I will let them borrow the Thunderbolt.

[passaloutre]

It certainly does sound best when turned above halfway, but the cascaded preamp helps it sound pretty good at lower settings. Would be perfect for a garage band.

[rafe]

I picked up a tube rectified thunderbolt in 2008? for 60 dollars and replaced the electrolytics and installed a 3prong .....Still sings. My first bass amp when I was 13 was a 420 harmony.....who knew ? Looks like all your questions have been addressed just wanted to chime in on a Truly "GREAT" amp, The only thing they got wrong was calling it a bass amp 

[passaloutre]

Not that I’m not completely thrilled with this amp, but being endlessly curious…has anyone tried a pentode in the V1 position of one of these? Something like an EF 86, or maybe even one of those triode/pentode combos so you could theoretically get both sounds?

[SoundmasterG]

Not that I’m not completely thrilled with this amp, but being endlessly curious…has anyone tried a pentode in the V1 position of one of these? Something like an EF 86, or maybe even one of those triode/pentode combos so you could theoretically get both sounds?

You would possibly have too much gain if you went with a pentode front end. Conrad Sundholm who co-founded Sunn back in the day was a friend of mine, and he came up with his Conrad Blues Jammer back around maybe 2006, based on a suggestion I had for a basic amp setup and a circuit idea, and some examples I showed him of real Supro Thunderbolts that I had repaired. The amp was based off a Supro Thunderbolt phase inverter and power amp, and he started off with a Fender 5F6A preamp in front of it. It was too much gain and would cause the amp to ultrasonic oscillate, and it would appear like the sound would cut out, but then the power tubes would start to red plate because it was trying very hard to amplify really high frequencies. I suggested to him to reduce the gain in the preamp, and after he did that, the amp sounded and worked well. It kind of became a local sensation in the Portland, OR blues jam scene, and I think he sold about 200 of them maybe. Even taking a tock Tbolt and putting the unused triode to work can result in too much gain and you have to pad it down a bit so it doesn't misbehave.


That said, if you didn't mind reducing the possible gain of the pentode, then it might be a useful replacement for the current preamp. I would use a pentode with lower gain though, like maybe a toned down 5879 rather than trying to use an EF86. Personally, I like Merlin's approach to use the pentode as the second stage instead of the first stage. It reduces the usual microphonics and is easier to get a good sound.

Greg