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Tweed Pro 5D5 Advice Gut Shots Added

Pedro58

Silver Supporting Member
Messages
5,815
I just got a 5D5 clone, bone stock, in a trade. It's a dark sounding amp. I've got some ideas about making it a little bit brighter, but I'm curious on what you all can offer?

My ideas:
12AY7's could be replaced all. or in part, with 12AX7's. I know gain would increase, but I don't know about top end.
Speaker could be switched to a 12" or a set of 10"... The current speaker is a Jensen 15. I'm not sure if it's alnico or ceramic. It looks mid 60's to me. I long ago decided I don't like 15's. So, it will definitely go.

Anything else?
 

Jeff Gehring

Gold Supporting Member
Messages
5,883
How does using the #2 input instead of the #1 input sound? It is NOT the typical relationship between the #1 and #2 jacks in this amp; the two triodes of V1 are paralleled and if you plug into the #1 input, you are driving both grids with signal. If you plug into the #2 input, you are driving one triode's grid while the other triode's grid is grounded.

Get the 15" gone, then maybe reevaluate. If need be, the 50nF coupling caps could be reduced, and/or a 25uF cathode bypass cap changed to something smaller.
 

Pedro58

Silver Supporting Member
Messages
5,815
There are four inputs. Do you mean #1 & #2 in each channel (bright/normal)? I was unable to plug into input #2 on both channels because the chassis is poorly aligned with the cabinet. I was only able to plug into #1 on each channel. I'll work on that later and get back to you... BUT the volume controls are somewhat interactive between the bright and normal channels, but only at extreme settings. If I crank the volume in one, it reduces the output of the the other.
The 15" is a mismatch. I think it's a bass speaker, maybe. The codes are for a '66 Jensen ceramic. I have a 2X12 loaded with Scumback M75/H75. I'll try that as as soon as I can. I also have a Weber 10" Silver. I can't remember if it's alnico or ceramic, but I'm going to try that, too.

It's a cathode bias design, right?
 

Jeff Gehring

Gold Supporting Member
Messages
5,883
Yup, it's cathode biased. Yes, for a given channel, the #1 input sends signal to both of the 12AY7's grids, (with its cathodes tied together, and plates likewise). The #2 input sends signal to just one of the two grids; the other grid is left undriven. That's if the cloner built it as a truly stock 5D5.
 

axpro

Member
Messages
628
This is a really REALLY simple amp., not a lot of tone shaping, and the interesting input jack wiring. I considered building one a few times. Of course, it all depends on how "bone stock" the original builder built it. Mallory caps versus Orange drops tend to sound a little "meatier" in my books, and lose a bit of brightness, The speaker is the big mystery here though. Not knowing what model it is, and if it IS from the 60's, it could be heavily broken in, sounding wonderfully smooth to someone who likes 15's but since you mention you aren't a fan, you are starting off in a losing position.

I think switching to 12ax7's loses some of the magic of the parallel 12ay7 sounds (I use them occasionally in Matchless type builds with a parallel gain stage) the ax7's will give you more gain and maybe a smidge of brightness, but I think you should start with the speaker, since you can feasibly just plug your amp into an external cabinet that is more your style. Start with a 1x12 or 2x10 and see if just a speaker change is all you need.
 

Pedro58

Silver Supporting Member
Messages
5,815
I'll get pics of the guts up and try speakers in the next few days... Thanks for the advice thus far, fellas!
 

Pedro58

Silver Supporting Member
Messages
5,815
I tried it through a closed-back 2X12 with Scumbacks (H75/M75, if memory serves) and it sounds like an amp now. Not quite as bright as I want, ultimately, but a HUGE improvement. Open-back combo might solve that, and I'm going to try a 10" in an open back tonight to see how that sounds. So, yeah, the 15" speaker sucked. I think it was a bass model... Pics forthcoming!
 
Messages
4,884
My thoughts...and you might not agree, but... A true vintage Jensen P15N is NOT a dark speaker. In a properly working Pro, it is a magnificent sounding speaker, ime. I have owned and resurrected a 5B5 TV PRo and a 5C5 Wide panel Pro. I have resurrected a '56 5E5A Pro. All 3 of those amps, despite their differences regarding the circuit and tubes, had full range frequency response...great highs, warm woody lows...great smoothness when pushed, articulation when clean. Fantastic, ime. The diameter of a speaker does not dictate frequency response.
Ime, if one of these circuits is dark, there is something about the circuit that is not correct, or...as you experienced with that one particular 15"...the speaker is dark on its own. Fwiw, the TV Pro above disappointed me after the recap. IT was very dark..no high end, ,muddy...unusable. I had replaced not only the electrolytics but all of those old paper and oil coupling and tone caps. So...what was left? The resistors. Ex: teh 250K plate resistors were measuring 470K and above out of the circuit. I ended up replacing every resistor that had had current flowing through them....that meant all of the resistors except the input resistors. The amp came to life....beautiful amp.
Speaker preferencess can be very subjective, but 15's can be dark or bright.....and 10's can be dull and muddy or thin and bright...same variations among 12's, ime.
 

Pedro58

Silver Supporting Member
Messages
5,815
Subjective is an understatement! I owned and played extensively a 5E5-A and hated the various 15" speakers I tried in it. It was a repro-clone, so I tried a lot of different types in it and settled on the P15N as the "best." But my ears preferred a 2X12 Greenback style cabinet. It is an old Fender Bassman 2X12 (solid pine, I think) loaded with a Scumback H75/M75 combo. I like that cab. I play my Hayseed 30 through it, and nothing else. (I've tried Celestion Blues, Silvers, Alnico stuff out the yin-yang... I just don't like alnico, either, not w/Vox flavors) I've played a 5C3 through a 15" and a 12", 2x10" Alnico... My Super Reverb kicked the snot out of two Vibroverbs I played alongside it, including a JBL loaded prize of some collector/tone-snob (whose opinion I still respect). I don't like 15-inch speakers... But I like the Tweed Pro circuit. I was very close to putting a 2X10 baffle in my 5E5-A clone when a buddy begged me to sell it to him. He still has it and it sounds lovely when he plays it. I'm happy with that outcome. With this Pro, a 5D5, I'm getting rid of the 15" speaker ASAP. The only thing left to debate is whether I can stuff a 12" and a 10" in the 18"X 18" space or just settle with 2X10's. And then settle on which ones. I like ceramic 10's and 12's, mostly. Maybe, just MAYBE, one alnico. There's no shame in that, right?

Anyway, I played the amp again tonight through a single Weber Silver 10A (Alnico) and it was plenty bright. But the Alnico tone is just too scritchy for me. The 2X12 Cab is thumpy as can be---Yes! The channels interact and the #2 inputs on each channel are quite nice. Working with those and the interaction of the two channel volumes and the tone really gives some variety. I'm no longer concerned that the amp itself is voiced too "dark." Maybe one Alnico, one Ceramic? Dunno. And it has Mesa 6L6's in it. I may still have some others... I'm wondering if a pair of KT66's would be okay in this amp. Cathode Bias? It might work!

Pics:
https://drive.google.com/file/d/0B0WPwlj1QcPwQl9naTZGZjVXNWs/view?usp=sharing
 
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TimmyP

Member
Messages
2,485
There's a .0001 cap between the plates of the PI. Changing that to .00001 might brighten it (or it might make it unstable).

Is it me *, or is the NFB quite odd on the 5D5?

* I started to say me not understanding, but I remembered that I don't understand PI circuits anyway.
 

Pedro58

Silver Supporting Member
Messages
5,815
I don't, either. But I can easily enough try your idea on the PI and know if it works. I might have time to try it this weekend, but I've got two gigs...

The guts, as simple as they are, intrigue me. As far as I can tell, the amp follows the stock 5D5 schematic. The #2 input jacks driving 1/2 of the 12AY7's is really cool, in practice. There's not just extra headroom, but another amp, almost, in the lower inputs. The tone and volume knobs interact differently enough to make it feel "new." I can't quite describe it, yet. The way the volumes interact, regardless of input, is new to me. If I crank the volume for the channel I'm not using, it reduces gain and I get a cleaner sound and less volume. Most of that reaction come in the last few "notches," 10-12 on the dial. Lower than that and the interaction just disappears. I've used amps that do this before but in the opposite direction. Anyway, I'm loving the interactivity of the channels, inputs and knobs. I haven't tried bridging channels like some other tweeds/Marshalls because I'm not sure what would happen. Is it safe?
 

Pedro58

Silver Supporting Member
Messages
5,815
Hogy was kind enough to let me know that bridging channels was safe, and that I could put kt66's in it if they did't overload the 250 Ohm resistor. I haven't tried KT66's yet. I only have a single from my Univalve days... But I did try bridging channels. Compression and gain in gobs. No headroom. Fun, but ultimately not that useful at gig volume. Really, the interactivity of the two channels unbridged is more interesting to fiddle with than bridging the two.

And I ordered a pair of Weber 10v and 10vs speakers for it. I don't like alnicos, but I wanted a Celestion voice. I'll give a tone report on them when they come in. What a cool amp! I'm glad it's not vintage or I'd be nervous fiddling with it.
 

kevmin

Member
Messages
1,439
You will like the Weber 10's, but the Eminence Legend 15 is a hard speaker to dislike. It just seems to do everything right! By the way, an original P15N Jensen is a fairly bright speaker. I think your problem with a dark tone could be greatly improved by putting a bright cap across one of the volumes. I would try somewhere between 100 picofarad and 180 picofarad. Fender used 100 and Matchless uses 180. I personally like 120 pf.
 

axpro

Member
Messages
628
Speakers are so subjective, Hope you dig the Weber 10's

I am on the other side, I like really dark speakers for the most part, and find that 10's are too snappy for my tastes. I've tried several versions, but never liked them. I prefer dark sounding 12's and 15s
 

Pedro58

Silver Supporting Member
Messages
5,815
Speakers are so subjective, Hope you dig the Weber 10's

I am on the other side, I like really dark speakers for the most part, and find that 10's are too snappy for my tastes. I've tried several versions, but never liked them. I prefer dark sounding 12's and 15s
If you'll pay shipping, you can have the 15 that was in it. I hate 15's. I can get a picture, too, so you can see what it is.
 

Pedro58

Silver Supporting Member
Messages
5,815
You will like the Weber 10's, but the Eminence Legend 15 is a hard speaker to dislike. It just seems to do everything right! By the way, an original P15N Jensen is a fairly bright speaker. I think your problem with a dark tone could be greatly improved by putting a bright cap across one of the volumes. I would try somewhere between 100 picofarad and 180 picofarad. Fender used 100 and Matchless uses 180. I personally like 120 pf.
I may end up doing this, anyway.
 

Pedro58

Silver Supporting Member
Messages
5,815
I put the 2 10" Webers in... Nice! I still want a bit more treble, so I'll probably put a bright cap on one of the volumes. I was looking at the NFB for the 5D5 and it's different from all the other tweed NFB circuits. There's a high value (1Meg) resistor. The other tweeds use a lower value. Also, and maybe I'm reading the schematic incorrectly, the feedback is fed into the 6L6/power tube instead of the 12AY7/preamp like those of other tweeds. Is this because of the unique preamp wiring of the 5D5? Anyway, if someone could explain why it's done differently, I'm all ears.
 

Jeff Gehring

Gold Supporting Member
Messages
5,883
It was Fender experimenting with designs with negative feedback loops on the power stage; fueled by the attraction of the reduced distortion NFB provides. Both the Pro and the Super "D" versions have that style of loop. Interestingly, they abandoned the NFB altogether for the 5E5, then it reappeared in the more familiar form where they loop back to just ahead of the phase inverter for the 5E5A variant (and was used in all of the three sisters: 5E5A, 5E7 and 5E4A).
 

pdf64

Member
Messages
7,060
http://bmamps.com/Schematics/fender/pro_5d5_schem.pdf
It's a very early global negative feedback loop for Fender, and may be the only one with a paraphase phase splitter?
There's not much gain in the loop, so little potential for it to do much; later loops included an extra gain stage.
A high value Rf feedback resistor is needed as it's connecting back to a fairly high impedance circuit at the power tube control grid.

The Rin is formed by the source impedance of the preceding stage, probably ~35k.
However, it's very unlikely there is enough open loop gain to support a closed loop gain of 1000/35 = 28.6.
The loop is most likely just providing a marginal amount of error correction.
If the 1M resistor was reduced in value enough for the loop to be more effective, then there would probably be too much load on the preceding stage and so insufficient signal voltage to fully drive the power tubes.

Apologies, control theory is a tough topic; I'm not sure that my explanation has been much help!
 
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Pedro58

Silver Supporting Member
Messages
5,815
Thank you, both. It is not a very "effective" loop, as far as cleaning up the distortion is concerned. Could I move the loop to the more common location (the plate of the PI) of the later amps? Or would that not work with the unique PI of the 5D5? Could I use a switch to change the location and keep the original? Lots of options, obviously. I want to gig the amp, if I can. A little more headroom would be nice for that application, but I can mike it, if need be. A little more top end for humbuckers would be nice, too. But I think I'll just put a bright cap on one of the volumes for that. Neither mod is totally necessary, but it's not a vintage piece, so I don't mind tweaking. But I must say that the stock circuit is pretty special.
 




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