Amp build for a first time builder?

[Reeltarded]

But lighter than a Bluesbreaker. THAT is one hernia waiting to happen.

God watches, and laughs.

[martin manning]

I have a couple of questions with the schematic

1) in the original schematic, some capacitors have 2 different numbers. For example, one of them is .02 - 400. what does this mean?

2) why is the capacitor in the oldschool ground switch called a death cap?

3) in your modification file, what does B+1G mean? How is B+1G different from B+1?

4) some of the values of caps/resistors are changed. why did you change these?

5)one of the resistors has a value '6k8'. I havent seen this format before. what does the 8 after mean?

thanks for the help

Asking specific questions is good, and organizing them is even better...

1) The capacitor markings are <value> <voltage rating> If there is no unit given, the capacitance is most likely in microfarads.

2) That capacitor is (for one position of the ground switch) coupling the AC line voltage to the chassis. If it fails shorted, the chassis and guitar strings will have 120V on them (the ground side of the guitar cable connects the chassis ground the guitar's ground). It was there to minimize hum at a time when polarized, grounded plugs were rarely seen, and it is no longer needed.

3) B+1G is the ground for the B+1 supply node, etc. Fender's original ground scheme was less than ideal, with all of the power supply filters grounded to the doghouse cover and the center tap of the high voltage winding grounded to the chassis near the PT. Its better (for minimizing noise and hum) to close the circuit for each power supply node with the negative terminal of its filter capacitor before or as it is grounded to the chassis and the current returns to the center tap. Also, high-current paths should be separated from sensitive parts of the circuit, like the input stage. Fender put a brass plate between the control pots and the chassis, and soldered the preamp grounds to that. You can see the outline of it in the Fender layout. In my scheme I daisy-chained the preamp grounds to the input jacks, and then use the jacks to ground to the chassis (B+4G).

4) The values in red are the ones seen in examples of amps from 1959, whereas the schematic is from 1957. As I mention in the layout notes, the reissue '59 Bassman also uses those values.

5) 6k8 is shorthand for 6,800, where the k serves as both the decimal point and the multiplier. Those little decimal points are small and easily lost.

One More) The 12AY7 used in the original Bassman is a lower gain tube (44 vs 100 for a 12AX7). Electro Harmonix is selling a 12AY7, so they are easy to get. Substituting a 12AX7 will drive the following stage a bit harder, and that is something that you can experiment with. 5881's are a higher rated (plate and screen dissipation) version of 6L6 and 6L6G. 6L6GC exceeds the rating of the 5881, so no reason not to use those.

I uploaded a new version of the modernized schematic- forgot to mention the relocated line fuse and the silicon diode replacing the selenium rectifier...

[skyboltone]

On the cap question. First number is capacitance second is voltage. You may substitute higher for lower but not the other way around.

[yoyohomieg5432]

One More) The 12AY7 used in the original Bassman is a lower gain tube (44 vs 100 for a 12AX7). Electro Harmonix is selling a 12AY7, so they are easy to get. Substituting a 12AX7 will drive the following stage a bit harder, and that is something that you can experiment with. 5881's are a higher rated (plate and screen dissipation) version of 6L6 and 6L6G. 6L6GC exceeds the rating of the 5881, so no reason not to use those.

what do you mean by plate and screen dissipation? At first you said that 5881's are higher rated and then you said 6l6 exceeds the rating of 5881. I'm confused. Which one are you saying to use?

[Firestorm]

One More) The 12AY7 used in the original Bassman is a lower gain tube (44 vs 100 for a 12AX7). Electro Harmonix is selling a 12AY7, so they are easy to get. Substituting a 12AX7 will drive the following stage a bit harder, and that is something that you can experiment with. 5881's are a higher rated (plate and screen dissipation) version of 6L6 and 6L6G. 6L6GC exceeds the rating of the 5881, so no reason not to use those.

what do you mean by plate and screen dissipation? At first you said that 5881's are higher rated and then you said 6l6 exceeds the rating of 5881. I'm confused. Which one are you saying to use?

6L6 is a metal tube, 6L6G is a glass tube. Similar ratings. 5881 was an improved variation. Not sure where the 6L6GB falls, but the 6L6GC is a significantly improved tube, ratings-wise.

[yoyohomieg5432]

what do you mean by plate and screen dissipation? At first you said that 5881's are higher rated and then you said 6l6 exceeds the rating of 5881. I'm confused. Which one are you saying to use?

6L6 is a metal tube, 6L6G is a glass tube. Similar ratings. 5881 was an improved variation. Not sure where the 6L6GB falls, but the 6L6GC is a significantly improved tube, ratings-wise.

\
so 6l6 and 5881 can be used interchangeably? what do you mean by ratings?

[skyboltone]

Heck they are ALL interchangeable! 6L6, 6L6G, 6L6GB, 6l6GC, 5881, KT66, EL34 and more even. It's just that they have different curves and different voltage ratings. It's just a bottle of electrons.

Dan

[Firestorm]

Heck they are ALL interchangeable! 6L6, 6L6G, 6L6GB, 6l6GC, 5881, KT66, EL34 and more even. It's just that they have different curves and different voltage ratings. It's just a bottle of electrons.

Dan

Voltage ratings and especially heat dissipation ratings for plate and screen.

[yoyohomieg5432]

so 5881's are favored over 6l6??

why is it that i've never heard of an amp using 5881s except for this? i see plenty of amps running 6l6

[M Fowler]

Because us old guys were using military/industrial tubes called 5881 in our Old Blackface Fender bassman amps back in the day.

Now days I use many various tubes 6L6GC, KT66, KT77, 6550, & 7581.

Some explanation on tube types here: http://www.vacuumtubes.com/6l6.html

Mark

[renshen1957]

so 5881's are favored over 6l6??

why is it that i've never heard of an amp using 5881s except for this? i see plenty of amps running 6l6

Hi yoyohomieg5432,

The 5881 was an industrial version of the 6L6. Industrial tubes were used in applications such as high vibration, high heat, computers (yes had them back then) and are the rugged version.

Fender switched over more than one of its amps to 5881s as supplied for a very good reason. Tubes that are upside down (heat rises so the area at the socket and pins are stress) in a high vibration (near speakers) area, and unlikely to be allowed to cool down before being bump around, into the trunk, back seat (of a car) or into the back of a station wagon (later van) to driven to the next gig or to a hotel tend to fail more easily.

Another reason, the 5881 is derated to 6L6 specifications but is actually a more powerful tube that can take the punishment of higher voltage, etc.
Another name for the 5881 was the 6L6wgb.

The tube is smaller than the coke bottle shape 6L6B, (called stubbies by some).

Check out Tube schematics at Schematic Heaven, Fenders more expensive amps had 5881 tubes for a short time

The Tweed 5F8 and 5F8-A (High Power Twin) did (by 6G8 the more powerful 6L6GC had debuted)

The Bassman kept the 5881 from the 5F6 through the 6G6-B series, doesn't upgrade to the 6l6GC until the AA165, Bandmaster 6G7-A has 5881 (but the earlier 6G7 has 6L6GC), Super 6G4-A has 5881 (6G4, 6L6GC) and the same thing happens with the Pro 6G5-A, Concert 6G12-A, Showman 6G14-A (the previous version has 6L6GC), Twin 6G8, etc.

An exception to the rule is the Vibrolux 6G11-A retains the 6L6GC

Either the price of the 6L6GC went up or Leo got a deal on 5881 tubes or discovered too many cases of left over 5881 in his warehouse. (Grins)

Tube choice is sort of a domain between the engineer, upper management (who perceive their average buyer, and the accountants trying to save money.

Gibson went to the 7591 tube and also the 6L6GC) as did Ampeg the 7591 as well as the 6L6GC and the 7027 (which Gretsch used on the Chet Atkins.)

Best Regards,

Steve

[martin manning]

Nice history lesson, Steve.

To answer the question what is plate dissipation, here's a description from the US Navy NEETS training manual:

"When electrons are attracted from the space charge to the plate, they are accelerated by the attraction. Their gain in speed gives them energy that causes them to strike the plate with a considerable force. As the electrons strike the plate, this energy is converted to heat. The plate must be able to withstand the associated increase in temperature. The maximum amount of power (watts) that a given plate can safely dissipate (as heat) is called the PLATE DISSIPATION rating. To find the amount of plate dissipation for a given tube under a particular set of plate conditions, use the following equation: Plate Dissipation = Ep x Ip"

The same concept applies to the screen grid. A quick look finds 6L6 and 6L6G rated at 19W max plate dissipation, 6L6GB at 22W, 5881 at 23W, 6L6WGC at 26W, and 6L6GC at 30W (all data from Tung-Sol).

I made another pass at the modernized layout. The notes are updated to better describe the extra ground leads coming from the doghouse.

[yoyohomieg5432]

hey guys, thanks again for the explanations. I think i'm going to start ordering things tomorrow. I think i will purchase what Martin suggested on page 4:

Transformers Triode Bassman Bundle w/ 2/4/8 ohm OT http://store.triodestore.com/bassmanbundle.html $168.83
Tubes Triode Bassman Re-tube set http://store.triodestore.com/5d6bassmanretube.html $94.95
Sm Parts Kit Mojo 5F6-A Boards, pots, jacks, resistors, caps, hardware http://www.mojotone.com/amp-parts/amp-k ... -Parts-Kit $127.00
Chassis Mojo Chrome Steel Chassis http://www.mojotone.com/amp-parts/Chass ... F6-Chassis $84.00
Cap Pan Mojo Filter capacitor board cover http://www.mojotone.com/amp-parts/Capac ... nformation $22.50

i really like this suggestion. just looking if there was any last minute suggestions, etc.

[yoyohomieg5432]

There's one thing that i'm a little confused with though.

The bassman tube set suggested my Martin includes 6L6 instead of 5881. It sounds like you guys prefer the 5881's, so should I ditch this tube set and just buy them separately?

p.s. if the bassman actually used 5881's, why is it that they are selling the 'bassman set' with 6l6? Is it the reissue that uses 6l6?

[skyboltone]

There's one thing that i'm a little confused with though.

The bassman tube set suggested my Martin includes 6L6 instead of 5881. It sounds like you guys prefer the 5881's, so should I ditch this tube set and just buy them separately?

p.s. if the bassman actually used 5881's, why is it that they are selling the 'bassman set' with 6l6? Is it the reissue that uses 6l6?

Yoyo. Dude. Build the damn amp. We'll fool with the tubes later.