Harvard build underway

[Niles]

Off to a slow start but I have the board made and partialy loaded, more parts in the mail. If I can get the photo links to work, I'll post photos, when I get a little farther along.

[guitar1amp2000]

This is my second Harvard, this one as a head only. Put an old Esquire in front of it and you've captured that Muscle Shoals sound.

[backporchmusic]

Sound clips!

[printer2]

Speaker?

[guitar1amp2000]

No sound clips, I don't have an interface.

Speakers are in a separate box. I've been running it to a pair of Peavey 16 ohm 12's.

[Niles]

[Niles]

I copied MeanGreenBlues board (thanks to him, by the way). Chassis and transformers on the way. Sourcing all these parts are a pain in the a$$. Thanks to master Jeep mechanic Tom Pierce for parts as well.

[hackworth1]

That's a good-looking board. Nice job.

[Niles]

[woodbutcher]

Hi Niles,

Thanks for posting this build. It is one that I want to achieve, eventually. Think I'll work on another Champ first to get that quiet thing (no buzz at higher volume) down first.

I have been doodling with MGB's board and the old Fender layout, trying to develop an overall plan. A few things are still baffling me.

Any chance you could show a 'back of board' shot to help clear things up?

Thanks,

Mark

[Niles]

I am still baffled by a few things myself. The bias circuit changes a few things and I believe he routed a few things differently from the Fender layout. Mean Green would help us both if he would chime in. I have studied his pics, countless times. If I have to start over mine will be as close to the original layout as possible.

[hackworth1]

There is a back of board pic here:

http://www.tdpri.com/forum/shock-bro...e-project.html

Any case, the 5E3 proluxe and the Harvard have pretty much the same connections.
The Harvard lacks the extra channel of the 5E3, but the Harvard includes a negative feedback (NFB) circuit.

http://taweber.powweb.com/store/5e3p_layout.jpg

The fixed bias needs a power supply - 50V or so - from the power transformer.

[woodbutcher]

Thanks, Dave.

I did have those pics stashed away in a file. Sometimes it is easy to overlook the things you thought were important enough to save! Decided to number the turrets, front and back to make things easier to see. I've also made a full size layout to figure things out on. See pics below.

The wiring for the trim pot section seems to be the most confusing for some reason. I fear that I am learning to do too much by the numbers rather than getting a better understanding of electronic theory. Hopefully a little will rub of by osmosis or something!

I think I understand where you're coming from, Niles. I am reluctant to post anything that might lead some one new down a blind alley, although I am sure than anyone with a little experience could tell I'm just a green horn!

Mark

[hackworth1]

Originally Posted by woodbutcher

Thanks, Dave.

I did have those pics stashed away in a file. Sometimes it is easy to overlook the things you thought were important enough to save! Decided to number the turrets, front and back to make things easier to see. I've also made a full size layout to figure things out on. See pics below.

The wiring for the trim pot section seems to be the most confusing for some reason. I fear that I am learning to do too much by the numbers rather than getting a better understanding of electronic theory. Hopefully a little will rub of by osmosis or something!

I think I understand where you're coming from, Niles. I am reluctant to post anything that might lead some one new down a blind alley, although I am sure than anyone with a little experience could tell I'm just a green horn!

Mark

Trim Pot for fixed adjustable bias:

Use 25 or 33K cermet pot with a 27K resistor to ground (power side ground) off the ground lug on the pot. The 27K resistor is a fail safe in case the pot fails.

You're not depending on the pot alone for all the necessary resistance but you have the pot so you can adjust the resistance to dial in the bias.

From the center lug (wiper) of the pot - solder one end of a length of 22ga wire and solder the other end to the junction of the two 220K resistors. (Number 28 on your diagram). Your power tube cathodes at pin 8 go by wire directly to the power side ground.

In a cathode biased amp, the junction of the two 220K grid resistors (at 28) goes to ground. Pin 8 on both power tubes goes by wire to a cathode resistor/bypass cap.

For fixed bias, you can use one or two small filter capacitors in the bias circuit. Two smaller ones are preferred over one big one in case of failure. 100V or so is good. 50V is too small. 2 x 10uF 100V are good for these Harvards.

Important: Filter Capacitors in a bias circuit are oriented backwards - with positive ends to ground. The bias circuit uses negative voltage. The negative voltage is the result of the orientation of the diode in the bias circuit. Positive voltage (typically 50V) comes from a dedicated lead off the power transformer. That single diode with its cathode end connected to that lead, changes the voltage from pos to neg.

[hackworth1]

http://www.el34world.com/charts/Biascircuits.htm

[Niles]

My Allen transformers aren't here yet but he sent me the color code wiring scheme and I think I have it figured out. This is my third build but wiring the power transformer is always scary for me I guess because it's the most expensive component in the amp. I ordered the cab today and I can hardly wait!

[Niles]

My 7 pin tube sockets arrived today and I will have extras as well as extra RCA 6AT6 tubes which have not arrived yet.

[woodbutcher]

Hi Niles,

I've been scrounging up parts as well. Did you get shields with the 7 pinners?

I am curious if they would be a good thing or not.

Mark

[Niles]

Nope no sheilds...

[Niles]

The Harvard crackled to life this evening. Mean Green helped me discover a couple of real dumass mistakes. I only had a cheap unmounted speaker in the shop but it seems to have a decent amount of volume. As soon as I get a chance to clean up some wiring, I'll take a picture. I will tell you now that I am not fond of the stainless steel chassis. I hope the cab shows up soon. I have an old Mojotone alnico 10 that I plan to use initially as it should be close to a Jensen vibe but I will most likely wind up with an Eminence Legend. I'll hook it up to my Mojo Pro build speaker tomorrow after work and that should be fun!