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Dirty-South Blues Harp forum: wail on! >
Thoughts on a '59 Fender Bassman RI
Thoughts on a '59 Fender Bassman RI
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Harpin J
1 post
Oct 06, 2013
7:57 AM
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Hi all,
I'm somewhat new to the group in that I've been lurking in the shadows for quite some time but never posted. That's about to change - for better or worse.
Okay, so I've been tinkering around with a very used '59 Fender Bassman re-issue (that I got for a song and couldn't pass up) because it just isn't giving me the sound I would like (isn't that always teh case when one is chasing tone). I've done some pre-amp tube swaps and those got it sounding closer, but still not quite "there." So, I finally got around to measuring the output tube plate current(s) and checking out the voltages on the thing. Ugh.
Now, my particular version of the re-issue Bassman lacks a bias adjustment potentiometer (but it does have the blue Eminence speakers and a solid pine cabinet) and I will have to (or would like to) put one in after getting the results from my electronics inquiry. Thoughts on the addition (or not) of the bias pot would be greatly appreciated...
The tube compliment in the amp for Test#1 is as follows (from left ot right looking at the back of the amp):
J-J Tubes GZ34S rectifier tube
ARS 6L6GC output tube (tube 1)
ARS 6L6GC output tube (tube 2)
RCA 12AX7 phase inverter tube (used, but tested good)
RCA 12AT7 pre-amp tube (used, but tests good)
RCA 12AT7 pre-amp tube (used, but tests good)
Plate voltage for output tube (measured from pin 3 to ground): 464V
Output tube cathode current (measured between pin 5 and ground while in standby mode) was witin schematic specs at: -54V (this was the same for both output tubes)*
Plate current for output tube 1: 46.1mA
Plate current for output tube 2: 23.6mA
Now, Test#2 used the following tube compliment:
NOS RCA 5U4G rectifier tube
ARS 6L6GC output tube (tube 1)
ARS 6L6GC output tube (tube 2)
RCA 12AX7 phase inverter tube (used, but tested good)
RCA 12AT7 pre-amp tube (used, but tests good)
RCA 12AT7 pre-amp tube (used, but tests good)
Plate voltage for output tube (measured from pin 3 to ground): 450V
Output tube cathode current (measured between pin 5 and ground while in standby mode) was witin schematic specs at: -54V (this was the same for both output tubes)*
Plate current for output tube 1: 32.2mA
Plate current for output tube 2: 16.4mA
*I measured the plate current on both of the output tubes simultaneously (I beleive that I have to adjust these values upward 10% to account for cathode current).
So, let the questions begin:
1) Would buying a matched set of output tubes bring the plate/cathode current closer to each other? The values I got seemed way to far out of line to be a good thing...
2) I like the extra volume I get with the 5U4G rectifier tube. I'm assuming that this is due to the lower operating voltages. Will running this tube (and the subsequent voltages) have any negative repercussions in terms of amplifier longevity?
3) Could I use 6V6s for output tubes with the operating voltage of 450V? Just a random question...
4) I will more than likely put a bias adjustment pot in the thing - I can't help but tinker. Would this allow me to use other types of output tubes?
5) I am also thinking about replacing the signal caps with higher values - 0.1 instead of the 0.022 values currently in the PCB. Thoughts?
I know some will be thinking, "Why not just sell that thing and get something that sounds great right from the go?" In fact, I am gathering the components necessary to rip all of the PCB guts out of it and replace them with an eyelet/turrent board construction. Which, I know, doesn't bolster my case. But in my defense, I've spent less money on buying the amp and getting all of the extra components than I would have buying a new Sonny Jr. or one of those beautiful Mission Harp amps made out there in Colorado. Plus, the learning experience has got to be worth something.
Any input would be great, and I apologize if this post is longer than is allowed by good web etiquette.
~Harpin' J
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5F6H
1673 posts
Oct 06, 2013
9:21 AM
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46mA is too hot really for a 6L6 in a bassman, with the tubes you have stick with the 5U4.
-54vdc is the negative grid voltage, not cathode current. The higher the value (e.g. -65v) the colder the bias & vice versa.
1/ Yes, matched tubes will bring the current closer together, but the current you run them at will also be critical maybe buy a couple of matched pairs at different current draws, say 30mA & 15mA (a decent vendor can work with the voltages you tell them). However, a degree of mismatch can be found to be warmer on the ear...you'll have to A/B test to see what you prefer.
2/ I assume by "extra volume" you mean the amp's volume goes higher before feedback, if you correct bias the amp should be louder with the GZ34. No, the 5U4 won't cause any issues.
3/ Possibly, if you disconnect 1 speaker and rebias (~17-20mA per tube?). Use JJ6V6, not NOS. The amp will be a little brighter, more compression than many 6L6/5881 types...but will still sound much like the same amp & pretty much as loud.
4/ YES! Put a bias pot in, not so much to use "different" types of tubes, but to optimise 6L6/5881 (possibly JJ6V6 types). KT66/EL34/6550 all need additional circuit mods, beyond bias, before they can be fitted.
5/ I wouldn't bother on the PCB, more likely to cause issues, this is the sort of thing to do with a new build/eyelet board amp.
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www.myspace.com/markburness
http://www.facebook.com/markburness
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Rick Davis
2515 posts
Oct 06, 2013
10:33 AM
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I agree with Mark:
4. Yes, put in the bias pot. That is where you dial the amp in to its sweet spot.
5. An eyelet board won't make the amp sound better. I've heard a Bassman amp with the conversion and it sounded no better than a well sorted Bassman with the PCB. Harpking amps have PCBs and they sound great!
As for your tube choices, I'd stay away from the 12AT7 in the secondary gain stage. I don't like the tone they make and they have a current output ten times higher than a 12AX7. They work better as phase inverters or drivers, not as a tone generator.
In my '91 Bassman RI I use 5751 - 5814a - 5751. I tried nearly every possible combination and permutation of tube types and this is the one that sounded best to me. Excellent tone and monstrous volume. I use a Kinder pedal to help tame feedback when I need to crank things up.
Hey, I have the big Bassman RI *AND* I have two "of those beautiful Mission Harp amps made out there in Colorado." Why limit yourself to just one amp?
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-Little Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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Harpin J
2 posts
Oct 08, 2013
8:49 AM
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Thanks for the input gentlemen!
While I would very much enjoy the tonal possibilities and characteristics of two or three (or four!) amps, I also very much value my marriage. (I know what you're all thinking: "Wienie.") That said, building, tinkering and/or compiling an assortment of harp amp parts and projects has not caused any duress in terms of my marital relationship (as of yet). Therefore I have all of these little projects that I tinker around with, hence the PCB-to-eyelet-board conversion I had been tossing around in my head.
Anyway, it sounds like I better get those 12AT7s out of there before something bad happens?! Maybe I'll try one in the PI stage and hear what it sounds like there. I do think I'll try to lay my hands on the tubes Rick mentions...never hurts to have a variety of tubes to play with. BTW Rick: do you use the same rectifier and output tubes as I mention? If not, what is your tube line-up?
If I do stick with the PCB in the amp, are there any other mods that would be recommended? I'm not shy about using a soldering iron...
I've seen some on the web but the directions and explanations didn't comport with PCB in my amp (differing resistor values and/or locations, etc.). Am I missing something? The schematic I have is good for my amp; was there a difference (aside from the bias pot) between other versions in the series?
Again, many thanks for the replies and I'll keep saving for a new Chicago 32-20, WEZO, SBJ, and too many others to list...
Last Edited by Harpin J on Oct 08, 2013 8:51 AM
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5F6H
1674 posts
Oct 08, 2013
9:23 AM
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Replace R39 with 82K-100K @ 3W. If you are looking at the article JimR and I wrote, we dual numbered parts in accordance with Weber schems & Fender.
The only parts differences between RIs in terms of component numbers in the circuit pertain to bias circuit filtering (a couple of revisions, not an area for modification), adjust pot & bias read resistor & diode on the LTD.
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www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by 5F6H on Oct 08, 2013 9:25 AM
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Barley Nectar
127 posts
Oct 08, 2013
5:23 PM
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Mark, do you have a link to that article?. Thanks...BN
Last Edited by Barley Nectar on Oct 08, 2013 5:23 PM
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tmf714
2082 posts
Oct 08, 2013
5:39 PM
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You can view the mods here-Bassman
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Harpin J
3 posts
Oct 09, 2013
7:04 AM
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I didn't mean to offend anyone or mis-speak. I was just confused because of the variety of mods out there. I mean, I read the article mentioned in the previous post(although I had no idea I was interacting with one of the authors!) and was confused because on my amp it seems as though the resistor to alter for the bias pot is R:41 not R:40 (as mentioned in the article). Then there's this from Gerald Weber's book: no extra resistor just a 10K-50K cerment pot soldered into place and siliconed to the PCB. Then there's this from Hoffman amps. Anyway, from all of the information it is easy to get confused. I'm just wanting to a) not kill myself, b) not burn my amp(s) up, c) learn a little something along the way, and d) dial my sound in.
With the help of this forum I might just be able to accomplish all of my objectives!
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5F6H
1675 posts
Oct 09, 2013
10:55 AM
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No offence taken J,
Gerald's bias pot mod & the LTD work by reducing the value of bias circuit load resistor (R41), this can only make the bias run hotter (rarely necessary for harp), unless you also reduce R40's value. So you usually need to reduce R40 to bring the bias cooler (if that is what you are after), the pot at R41 then allows you to fine tune/bring it back up, if desired.
Making R40 smaller, lowers the bias.
Making R41 smaller raises the bias.
Lower voltage rectifiers (like 5U4G) lower the bias.
5AR4/GZ34 run hotter than 5U4 (~+5mA), SS runs the hottest (~+5mA over the GZ34). It's often possible to pick an average that works acceptably, with the same set of power tubes, with all 3 rectifiers (or very well with the 2 closest options) if you don't want to be getting the meter out all the time.
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www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by 5F6H on Oct 09, 2013 10:59 AM
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tmf714
2085 posts
Oct 09, 2013
11:03 AM
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I had great success with the Hoffman kit-I think I may have changed two resistors after the point-to-point install. I used all vintage NOS tubes. The amp was dead quite at 32ma and ripped at 8 on the volume with no feedback.
Last Edited by tmf714 on Oct 09, 2013 11:05 AM
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5F6H
1676 posts
Oct 09, 2013
11:17 AM
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Hoffman's set up is as per Gerald Weber's, again the adjustability is on the load resistor. His board has an additional bias range resistor, this helps correct voltages because of the RI's cooler bias supply. I didn't mean to imply the Hoffman kit (or other quality board install) wasn't worth doing, I had one too, it's a lot of work & cost, best installed by an experienced tech (I have seen bad installs, beyond Hoffman's control - he supplies detailed instructions but not everyone reads them, that required amps to be gutted & rewired for a third time!). Circuit is an earlier 5F6A than the RI, both have their devotees.
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www.myspace.com/markburness
http://www.facebook.com/markburness
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tmf714
2086 posts
Oct 09, 2013
12:06 PM
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Doug has a list of preferred installers for the kit-my tech at the time was Chris Davis-guitar tech for Hall and Oates. His name is on the list,and he was more than qualified to do the install,and at a great price as well. I was fortunate to know Chris-I can see how things can get misconstrued-the instructions are quite laborious.and along the way,Chris had me stop by to get his thoughts on certain"modifications" and edits to Dougs installation instructions-Chris knew I was intending on using the amp for harp only.
He also did a "before and after" recording. I played the stock amp before and after the mods-quite astounding to sat the least-Chris was floored by the difference in usable volume,headroom and cut,not to mention much better anti-feedback properties.
Last Edited by tmf714 on Oct 09, 2013 12:14 PM
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