'68 Fender Showman - circuit question

[Jason Summer]

I recently acquired a Fender Showman from a local shop that, according to the chassis stamp, was built in the 8th week of 1968. It appears to have a relatively unusual, cathode biased, transitional circuit (with some updated components).

I haven't been able to find much information on this specific circuit - before things went to mixed bias (I understand this happened in May of '68). I'm curious if anyone has any information or experience with this circuit in these higher-wattage amps? Was it such a short lived design because it proved to be problematic, or was there push-back from dealers/customers because it was such a departure from the fixed bias that people had become accustomed to?

showman chassis.jpg

showman bias 1.jpg

showman bias 2.jpg

[Raybob Bowman]

Someone modified the circuit. It originally had a bias pot in that hole to the right of the power transformer. Schematic is here: https://el34world.com/charts/Schematics ... _schem.pdf

If it were my amp, I would change the circuit to AB763 circuit, before it was changed to the AC568 back in May '68

[Jason Summer]

Thanks for the reply Raybob - I also thought that it may be a modified circuit, but someone over on the Telecaster forum also has a Showman with the same circuit, the same large sandcast resistor, from the same year/week/assembly tech as mine. Seems like too many things to just be a coincidence. https://www.tdpri.com/threads/68-fender ... l.1064744/

[Raybob Bowman]

Every known schematic of every Fender Showman is found here. I don't believe one of them has cathode bias.

https://el34world.com/charts/Schematics ... matics.htm

[J Fletcher]

I have heard of these amps in the past , so I suspect they came cathode biased from the factory. Ran very hot is what i also heard . Other than that , I can't shed any light on this amp . Something that was tried for a short period, and then abandoned.

[Stephen Cowell]

The original Fender circuit (CBS!) was 'part-cat-bias'... some DC was introduced at the cathode of the finals to give some self-regulating aspect to the bias. Pretty sure this was done as a cost-saving, to allow use of final tubes that were farther from matched than before. The mod the OP shows is full cathode bias, with the grid bias board disconnected. The original 'C' skem below shows the much lower cathode resistors and balancing, instead of bypass, capacitors.

Note that the OT transformer polarity is reversed... the NFB comes into the other grid of the LTP PI. If you don't fix this when modding the circuit (say, to return to normal AB763) you run the risk of burning the OT... found this the hard way back in my BF Bassman modding days.

https://schematicheaven.net/fenderamps/ ... _schem.pdf

[Dave Grafe]

Someone modified the circuit. It originally had a bias pot in that hole to the right of the power transformer. Schematic is here: https://el34world.com/charts/Schematics ... _schem.pdf

If it were my amp, I would change the circuit to AB763 circuit, before it was changed to the AC568 back in May '68

What he said

[Raybob Bowman]

That brand cap used as bias cap "MOD" was not even in existence in 1968. Probably won't find that brand cap anywhere else in any 60s Fender amp, unless it was replaced by someone else later.

[Jason Summer]

Thanks for the responses, I appreciate the advice!

[Dave Mudgett]

Yes, someone changed that large MOD capacitor, it is not original. But if you look at this thread that the OP linked to on TDPRI - https://www.tdpri.com/threads/68-fender ... l.1064744/ - you will see that there are at least two other examples of cathode-biased '68 Showmans out there in the wild, with that exact same cathode biasing scheme with the same power resistor and large cap. The one at the top of that thread has the original brown electrolytic where the MOD cap is on the one Jason's showing us. And I agree with him that I see no indication of a bias pot being there. Why is the hole there? Well, would they make a bunch of separate chassis without the hole just to try out a new design? I doubt it.

My bet is that these are, in fact, actual transition '68 Showman amps where they were trying out a cathode-bias approach. Obviously, they didn't stick with this, and moved to the mixed fixed/cathode-biased approach on some of the new amps. But Fender is well-known to have experimented a lot with amp designs over their entire history. Close to annual changes were pretty routine. And '68 is when the new CBS managers really started making wholesale changes in both guitars and amps. The objective was generally to speed up production and reduce costs. And remember the ill-fated solid-state Fender amps? They did all kinds of crazy stuff.

I totally agree that it makes absolutely no sense to cathode-bias an 85-watt amp - what were they thinking? I don't generally like the mixed fixed/cathode-biased amps. I almost always prefer changing the higher-wattage silverface amps back to blackface specs. But that doesn't mean that some weird examples like this don't exist. I've seen all kinds of stuff in old Fender amps. Gibson, even more so. Not everything made it to the published schematics.

[Raybob Bowman]

Cathode bias might be a cool sounding mod for guitar, more dynamic in overdrive. Tweed Bassman and Tweed Deluxe were cathode bias, but fixed/adjustable bias would give much more headroom overall.

[Dave Mudgett]

Cathode bias might be a cool sounding mod for guitar, more dynamic in overdrive. Tweed Bassman and Tweed Deluxe were cathode bias, but fixed/adjustable bias would give much more headroom overall.

I'm not arguing for cathode bias in a silverface Showman, LOL. Especially not for clean. I'm simply stating my belief that the '68 examples the OP is discussing may have been originally set up that way.

BTW - tweed Deluxe through at least 5E3 was cathode biased, but tweed Bassman from circa 54/55 or so was fixed bias. My '57 5E6/5E6A (forget which) 2-input Bassman - fixed bias. The much venerated 5F6-A, definitely fixed bias. No bias pot, just fixed resistor/cap - but definitely fixed bias. And my '57 sounds great for pedal steel, to my tastes. And possibly the best guitar amp I've ever heard, if I can play loud enough to let it open up a bit.

[J Fletcher]

So , how many volts are dropped across that 165 ohm cathode resistor ? And the 6L6 plate voltage ? Just curious , thanks .

[Dave Grafe]

Here's the AB763, most voltages are essentially unchanged and do vary from amp to amp. The schematics say +460-470 vdc at the plates but I have seen as high as +485 vdc on some '64's. Other nominal voltages are marked.

[Stephen Cowell]

Here's the AB763, most voltages are essentially unchanged and do vary from amp to amp. The schematics say +460-470 vdc at the plates but I have seen as high as +485 vdc on some '64's. Other nominal voltages are marked.

That plate voltage goes up and down with the bias setting... hotter final bias can actually sweeten up preamp stages by bringing the plates down a dozen or so volts. It all interacts.

[Jason Summer]

So , how many volts are dropped across that 165 ohm cathode resistor ? And the 6L6 plate voltage ? Just curious , thanks .

Thanks again for your responses, everyone. When I have a chance to dive back into this project, I'll calculate the voltage drop and the 6L6 plate voltage.