Bassman 59 mods for harp

[Leevi]

I'm modifying a vintage Bassman 59 (not re-issue) for a harpist.

There has been added rectifier switch to the amp in order to select either SS- or tube rectifier.
The b+ is with SS 480v and with tube (4UGB) 427v.

I have found a list of changes that are recommend to be done:

- 6L6 -> 5881 (e.g. Sovtek 5881 WXT)
- bias to ~15mA
- use 12AY7 instead of 12AX7
- for RI http://www.bigharp.it/bigharp/home.php?status=41&linkaa=home

We also noticed when playing through an analog delay that the
harp sound improved remarkable. This is probably related to input impedance
issue of tube amp and acoustic microphones?

If you have any experience and/or tips of using harp with Bassman please reply.

/Leevi

[Willabe]

I started out playing blues harp before guitar. I have not tried any of these but I do think some or all will work fine for modding a guitar amp to a blues harp amp.

Yes, use lower gain preamp tubes.

You can also try dropping the 1st preamp tubes plate voltage to ~120vdc or even a little lower. (Increase the B+ dropping R.)

And believe it or not try reversing the speakers polarity so that the speakers "throw" is backwards 1st. Plus try running the speakers at a different impedance. If OT has an 8 ohm speaker tap try wiring the speakers at 4 ohm or 16 ohm.    

Kevin O'Connor has a circuit mod in TUT5 that he calls a continuous push/pull to SE control, something like that. I lost my copy but it has to do with putting a pot in the pull side that dials out the signal to the pull sides tube. It still leaves the pull side tube biased up and so the OT still sees the same load and the PP noise cancelation is still working.

You can also try knocking the PI out of balance;

Gerald Weber has written (4th book, Tube Guitar Amplifier Essentials, section 11, Harp Amp Secrets and Tips, Doug sells it in his store) about using a self-balancing paraphase inverter (not the fixed paraphase  ) and replacing an R that's common to both (he doesn't say exactly what R) circuits with a pot and it throws off the PI balance. He said it fattens up the tone by making the signal waveform non-symmetrical. He calls it a "waveform symmetry control.

"Then he added this;

"I found if I connected the speaker reverse polarity with respect to which half of the waveform is bigger, then the threshold of feedback goes up. This means the amp can be turned up to ridiculous volume levels before acoustic feedback occurs."

Hopefully one of the deep theory guys will explain in better detail what and how to do it.    

There has been added rectifier switch to the amp in order to select either SS- or tube rectifier.
The b+ is with SS 480v and with tube (4UGB) 427v. - 6L6 -> 5881 (e.g. Sovtek 5881 WXT)
- bias to ~15mA

In the same chapter he says he used a GZ34 run in cathode bias, biased up the 6L6GC to 70mA per tube and refers to class A. (He does not say the 6L6GC plate voltage.)  Harp players almost always want sag/compression for sustain so going with a SS rectifier with cold -bias is going to take away any sag/compression. If he wants clean then just play through a mic into the PA system.  


                Brad    

[HotBluePlates]

I'm modifying a vintage Bassman 59 (not re-issue) ...

PLEASE post what stock cap values you find in the tone stack (my info says they will not be 250pF 0.02uF, 0.02uF).

Yes, use lower gain preamp tubes.

You can also try dropping the 1st preamp tubes plate voltage to ~120vdc or even a little lower. (Increase the B+ dropping R.)

And believe it or not try reversing the speakers polarity so that the speakers "throw" is backwards 1st.

I've heard each of these as well. The speaker phase things really begs having the player there testing out the amp to see if it's worthwhile.

... Plus try running the speakers at a different impedance. If OT has an 8 ohm speaker tap try wiring the speakers at 4 ohm or 16 ohm. ...

Original OT doesn't have multiple taps. Just a single 2Ω tap for placing all 4 speakers in parallel.

That said, he can try 8Ω series/parallel wiring pretty easily.

Kevin O'Connor has a circuit mod in TUT5 that he calls a continuous push/pull to SE control, something like that. I lost my copy but it has to do with putting a pot in the pull side that dials out the signal to the pull sides tube. It still leaves the pull side tube biased up and so the OT still sees the same load and the PP noise cancellation is still working.

One of the 220kΩ resistors from the bias supply to a 6L6 grid would be replaced with a 250kΩ pot. Or after the 0.1uF coupling cap you add a pot wired as a typical volume control, with the wiper going to a 1uF cap. The other side of the 1uF goes to the existing 220kΩ bias feed resistor and pin 5 of the 6L6.

This is like a post-phase inverter master volume, but only on one side of the output section.

I have no idea if this change would be worth it. I'd tack it in the circuit for the player to try before trying to figure out how to actually install it in the amp.

You can also try knocking the PI out of balance;

Gerald Weber has written (4th book, Tube Guitar Amplifier Essentials, section 11, Harp Amp Secrets and Tips, Doug sells it in his store) about using a self-balancing paraphase inverter (not the fixed paraphase  ) and replacing an R that's common to both (he doesn't say exactly what R) circuits with a pot and it throws off the PI balance. He said it fattens up the tone by making the signal waveform non-symmetrical. He calls it a "waveform symmetry control.

"Then he added this;

"I found if I connected the speaker reverse polarity with respect to which half of the waveform is bigger, then the threshold of feedback goes up. This means the amp can be turned up to ridiculous volume levels before acoustic feedback occurs."

This amp almost certainly has a long-tail inverter. I'd advise against major circuit changes like going from long-tail to paraphase, given this is an original 59 Bassman.

A similar control to unbalance the long-tail could involve replacing the 10kΩ tail resistor with a pot wired as a variable resistor. Or maybe the master volume type control some folks use where they replace the 470Ω bias resistor with a pot wired as a variable resistor (to get much more resistance).

With all this, I'd probably look to unhook the feedback circuit in this amp, at least for the player to try.

As for the speaker polarity bit, again, the player has to try it, with or without unbalancing the phase inverter. Just like a polarity switch on an acoustic guitar DI: whether it kills feedback depends on the specific venue and application. But Weber's description means "bigger output signal half-wave = half-wave when the speaker sucks inward." I imagine whether that will kill feedback depends on where the player stands relative to the amp.

[HotBluePlates]

... he says he used a GZ34 run in cathode bias, biased up the 6L6GC to 70mA per tube and refers to class A. (He does not say the 6L6GC plate voltage.)  ...

30w / 0.07A = ~428v (I'm guessing Weber's talking about typical Bassman supply voltage of ~430vdc)

I dunno how you run class A and not cook output tubes without dropping plate or screen voltage, but whatever.

... Harp players almost always want sag/compression for sustain so going with a SS rectifier with cold -bias is going to take away any sag/compression. ...

Fastest, least-invasive way to get more sag is to use higher-value screen resistors. Try values between 2-5kΩ. More resistance increases compression near maximum output power, and also reduces maximum output power. They limit plate current when screen current rises significantly, which happens when you try to push maximum output power.

- 6L6 -> 5881 (e.g. Sovtek 5881 WXT)

When harp player talk about using 5881's, they don't mean the Sovtek tube.

They mean the 50's Tung-Sol 5881's that came in the amp originally. I don't know how critical the amp's tone is to the use of those, or how the new Russian-made "Tung-Sol 5881's" compare sonically. They certainly look similar.

We also noticed when playing through an analog delay that the
harp sound improved remarkable. This is probably related to input impedance
issue of tube amp and acoustic microphones?

What microphone is he using? What kind of delay?

I would think a tube amp input would be higher impedance than a delay pedal (if you used a pedal). I'm not sure if different impedance would make a difference in the mic output (probably depends on the mic).

I advise trying any/all tips one at a time with the player testing them out. I don't play harp and can't begin to guess how a harp player might want a guitar amp to change.

[Willabe]

I'm modifying a vintage Bassman 59 (not re-issue) ...

Original OT doesn't have multiple taps. Just a single 2Ω tap for placing all 4 speakers in parallel.

I missed that, sorry.

... he says he used a GZ34 run in cathode bias, biased up the 6L6GC to 70mA per tube and refers to class A. (He does not say the 6L6GC plate voltage.)  ...

30w / 0.07A = ~428v (I'm guessing Weber's talking about typical Bassman supply voltage of ~430vdc)

I dunno how you run class A and not cook output tubes without dropping plate or screen voltage, but whatever.

Yeah he starts the chapter talking about an old National wood cab amp from the 40's, 2x6L6G's, field coil, run in class A. A guy named Paul Orta from the Kingpins bought the amp from Jimmie Vaughan and Paul brought to Gerald for him to clone a louder and quiter verson because he loved the sound of the amp for electric harp.

He then latter talks about changing a SR to cathode bias and how the power tubes see less voltage. Then he says you can now see how because of the voltage your dropping across the KR how you can then bias up the amp that high, never saying how low the plate dcv was to be able to get to 70mA per tube.  

I only brought it up because Leevi said he took the bias down to 15mA.

Tone Quest Report mag. did an article years ago written by a harp player who gave tips on how to get an amp to play better for a harp player. I'd have to dig through a few hundred back copies to find it but it was mostly little things like, lower gain preamp tubes and what I thought at the time and still do, lower biased output tubes. So I guess you'd have to try it both ways.      

I knew and saw Big Walter, Jr. Wells, James Cotton, Cary Bell, Sugar Blue and Billy Branch (most of the old harp men didn't know my name but they knew my face, that I played guitar and would talk with me if I went up to them) play live in small clubs any where from a lot to a few times and none of them played through an amp at that time, even though they used too. Sugar Blue was the only guy that always played through an amp for his gigs, but it was a single 12" Mesa Boogie. Only the young (mostly white guys) bothered to play through an amp. Blowin harp into a cupped mike will distort the mike plenty.  

I don't play harp and can't begin to guess how a harp player might want a guitar amp to change.

The biggest problem is feedback from the mic. That's the problem with bigger amps like a tweed bassmen. Smaller amp then run it through the PA. I know it's cool to have a bassmen on stage to blow harp through but you have to tame it.

Little Walter was and still is king of electric blues harp. Mind you LW also played most if not all of his most famous songs where he really lets the note hang while slowly bending it a long time in a low key harp, big reeds. He like all the old guys played through what ever they could get their hands on and keep. LW did use a echoplex in the studio at Chess but I doubt that he played out at any clubs with one.       Old green bullet type mics were cheap so they used them. Cheap used low power tube amp (low B+, weak power supply, low gain preamp tubes, cathode bias) with a bullet mic cupped in your hands and you get lots of compression and a singing sax/horn type sound. Pretty much it.

Whether LW or any of the old blues men played through a tweed bassmen in the studio or out at gigs is really up for debate. The few guys that are still alive probable didn't care who was playin through what and if they did time fades the memory.

Out of all the bands/players I saw and years I hung out listening to the old blues guys I remember seeing only 1 Fender tweed amp and it was Buddy Guy, his old tweed bassmen that he recovered in kitchen cabinet shelf paper.         Most of those guys in the 80's/90's played black face Fenders, SR and TR. They liked the reverb and they wanted a cleaner not dirtier sound. Those amps had plenty of dirt for their style when turned up. Plus better reliability.      

It's been said by Derrick Trucks that Duane Allman was doing LW's slow hanging bends on slide, that's where he heard it and tried to copy it with slide. I believe it.    

Big Walter was the 1st blues guy that let me sit in. He was a character and when you mentioned any other blues harp player he'd say "I taught him."          And it was true.


              Brad      

[Leevi]

PLEASE post what stock cap values you find in the tone stack (my info says they will not be 250pF 0.02uF, 0.02uF).

Unfortunately the amp is not anymore at me so i cannot check the values.

We tested the amp with different tube combinations by changing both the preamp tubes (12AU7, 12AT7, 12AY7, 12AX7)
and power tubes (6L6, 5881). We also turned the bias to the lowest possible value we could which was 27mA.
The best result was achieved with 5881s and by using 12AX7 in the first amplification stage. Rectifier was tube which produced
about 440V B+ after bias was turned down. However we didn't noticed any difference by changing the rectifier to SS which
gave 480V B+. All in all the sound was still far away from the sound what the harp player was looking for. The analogue delay
corrected the situation close to the wanted sound. Then we tried couple of other amps one with ECL82 and the second one with EL34s
and both of them worked much better than the Bassman.

/Leevi

[Willabe]

Here's a link that might help you. And check out his link page also.

http://www.harpgear.com


             Brad      


Edit;  I can't seem to get the link to show the whole page, it's leaving out the far left that has the site menu, which includes his link page.    

EDIT... fixed link

[Leevi]

Thanks for the link Willabe.

What microphone is he using? What kind of delay?

There were several microphones of different types all vintage from 50's/60's (rounded form).
The delay was one of the cheapest you can find Behringer VD400:

http://www.behringer.com/assets/VD400_P0508_M_EN.pdf

/Leevi

[HotBluePlates]

The 500kΩ input impedance doesn't seem different enough from the amp input to make a difference.

The 1kΩ output impedance may make a difference, especially in driving the cable to the amp. I'd bet that pedal always has the signal buffered regardless of whether the delay is applied or not.

[jjasilli]

Here's the thread form my bullet mic project:  http://www.el34world.com/Forum/index.php?topic=2474.0  Can't believe it was 6 years ago!

Old fashioned, dynamic, hi impedance mic's don't sound as "good" as modern mic's and have a limited frequency range.  But so do harmonica's.  If the goal is to play Show Tunes on a chromatic mic, then a modern lo imp. mic is the way to go.  Opposite for a traditional blues harp sound.  Leevi did not specify, but if he's plugging into a Bassman, I'm guessing Blues Harp.

[smackoj]

I have re-read G. Weber's "how to modify for harp" article and noticed some common themes. Gerald started his quest by doing a break-down of an early 40's National tube amp (owned by a well known harp guy) that ran 2-6L6G tubes in Class A push/pull. Early National amps were built by Valco I believe? Gerald's recommendations seem simple enough but I don't get the feeling an orig. '59 bman is a good prospect for harp 'changes'. Pretty darn good guitar amp w/o changing voltages and coupling caps and phase inverter scheme.

I did a googee search and found this thread on a forum "best harp amp I ever heard"  which talks about an early Premier Twin 8 amp. If you follow the links on the forum you find schemos for six or eight of the early Premier amps. (They all have very similar circuits to the early Valcos and Nationals which tells me sumthin).

here ya go:  http://music-electronics-forum.com/t8066/

smacko jack