68 Fender Showman, fixed bias?

I am ordering a new set of tubes for my head and keep seeing things about bias adjustments. Do older Fender amps have such a thing? I dont see a pot for bias adjustment on the back of the amp.

Just wondering if its something I should worry about.

Yes, it's fixed biased.

it is indeed fixed bias, but it is adjustable. The bias adjustment pot is inside the amp. You may have to remove the chassis to access it, but it's possible you can access it from the underside of the chassis. You need to look for a hole in the bottom of the chassis near the tremolo pots and power transformer.

Ahh, thx guys.

A '68 Dual Showman may have one of two different biasing schemes. IT all depends on what circuit is in the amp. IF the amp was built before the AC568 schematic was introduced, then the amp has a true bias voltage adjustment circuit in a true fixed bias scheme. This would be the AB763 circuit. IF the amp has the AC568 circuit that was introduced in May of '68, then the amp has a mixed fixed-bias/cathode-bias scheme with a balancing bias circuit in which the voltage cannot be adjsuted but the balance of the voltage application to each side of the OT primary is adjustable. IF you could post a picture of the circuit board near the power tubes we could tell you what you have there. Adn...no, the tube chart is not an accurate clue as to what circuitis in the amp because Fender used old tube charts until they were gone regardless of what schematic was used in building the amp.

it is indeed fixed bias, but it is adjustable. The bias adjustment pot is inside the amp. You may have to remove the chassis to access it, but it's possible you can access it from the underside of the chassis. You need to look for a hole in the bottom of the chassis near the tremolo pots and power transformer.


I recently got a Li'l Dawg Wonder Dawg (Jim's take on a blackface deluxe). It has a bias pot which is best accessed by pulling the chassis which I belive will be the best way in your case. Also bought a bias tester that you plug in between the power tubes and your digital multi meter that comes with great instructions making the job easier for the un-initiated.http://www.amp-head.com/product_info.php?cPath=21&products_id=31 Seems like with any new venture it's a good idea to learn to DIY. Good luck!:mrgreen:

Off topic but while I was looking around in the amp I found a Raytheon 12AX7 preamp tube. That lil guys sells for $199.95 over at tube depot. Somebody had a new Electro Harmonix in V2 so I pulled it out and stuck the Raytheon in V2 and the EH in V5.

There was also an old Sylvania 12ax7, I put that into V2.

I'll get a pic of the guts later, everything is packed up for tonights gig.

I'm thinking I got a good deal on this amp for $450
Unlike the 64 blackface showman with no reverb that I paid $1040 for.

I actually like the silverface sound better for my brand of twang. Seems to have more bite to my ear than the 64. Could be the tubes? Who knows, and I havent even had my amp tech recap the 68 yet. Its a bit hissy but sounds great.

Off topic but while I was looking around in the amp I found a Raytheon 12AX7 preamp tube. That lil guys sells for $199.95 over at tube depot. Somebody had a new Electro Harmonix in V2 so I pulled it out and stuck the Raytheon in V2 and the EH in V5.

There was also an old Sylvania 12ax7, I put that into V2.

I'll get a pic of the guts later, everything is packed up for tonights gig.

I'm thinking I got a good deal on this amp for $450
Unlike the 64 blackface showman with no reverb that I paid $1040 for.

I actually like the silverface sound better for my brand of twang. Seems to have more bite to my ear than the 64. Could be the tubes? Who knows, and I havent even had my amp tech recap the 68 yet. Its a bit hissy but sounds great.

What tube is in V1? That is the one that will most shape the tone.

What tube is in V1? That is the one that will most shape the tone.

Mick, in the two channel Fender amps of that era; V1 is the preamp tube for the Normal Channel while V2 is the preamp tube for the Vib Ch. So, it depends on what channel you are in as to whether or not V1 affects that channel in a direct manner. IF your Lil Dawg is a 'faithful' clone of a Deluxe, then it will be laid out in this manner, too. AS to whether or not that is the case, I don't know.

I use channel #2 only.

V1 has an old 7025 in it. The name has worn away so not sure of the brand.

DeepSouth, you may or may not know this already. IF you want a bit more ooomph out of teh Vib Ch, you can pull V1. V1 and V2 share a cathode bypass circuti on pin 8 of each tube. When you pull V1, the bias and therefore the gain is changed in the Vib Ch.
When working on some of these types of FEnders, I like to separate these cathodes. This allows one to do some interesting things....like make the Norm ch hotter and more Tweedy by changing the tone caps, the slope resistor and that resistance for the new bypass circuit for that pin 8. IF one does this, one can take the 820 ohm resistor from the shared bypass circuit and use it in the NOrm ch. This increases gain...just as in the scenario where you pull V1 to 'heat' up the Vib ch. To maintain 'stock' bias and gain in the Vib channel, one needs to use a 1.5K bypass resistor with a new cap for the Vib Ch's pin 8. This makes these amps much more versatile, imho. Add an A/B pedal, and you have a useable 2 channel amp...with very different sonics in each channel. I like to put in an adjsutable NFL circuti, too. The results will surprise a person.
IF the amp is exc cond all-original, I might question doing this type of thing, but hten again it is all easily reversible.

Interesting Wally, how much more gain we talkign about here? I play mostly country and like to use a classic twang as much as I can. I kick in pedals for modern stuff.



Naw the amp is not 100% stock, somebody bypassed the master vol. I wont know if anything else was done till I let my amp guy go over it next week.

After checking the date codes on the transformers *606-4-37* I think this is a 74, not a 68 like the guy I bought it from thought. Somebody has tinkered in the amp. The master vol has been bypassed and I see a few new caps in there. I'm taking it to my amp guy next week to check it out and clean it up a bit. Mike Kennedy does my amp work, hes the guy that builds Komet amps. If you want to see artwork, look inside a Komet.

I giged it last night and it did great. I think my 64 has a bit more depth? Harmonics? something....but it has no reverb and this amp does, and is sounds soo much better than a verb pedal. The 64 is also in alot better shape than this one so if it gets dropped or dinged on a gig I wont freak out. At gig vol I really don't think anybody would ever be able to tell the diff between the two amps.

The bias has been Bf'd back to a bias voltage adjustment circuit as opposed to the balancing circuit of th '68 and later amps. IT has fresh bypass electrolytics and a fresh bias cap.
The MV hole is a great place to put an adjustment pot for a variable NFB loop.
M. Kennedy will be able to fill you in on options.

Thx Wally, all ths is makeing me want to learn more about whats going on under the hood on these amps.