AB763 help please.

[Enzo]

True, just moving the point where some sire is grounded a couple inches to the side can make a real difference in hum, for better or worse.

[Stratz]

Hi Tom. Since I dont have the luxury of the brass ground plate as the older Fenders have I'm using a tried and true grounding scheme from an amp builder friend in Belgium which employs terminal strips.
I do trust his years of building experience and knowledge of Fender circuits. He uses it on all of his Fender builds and says it works extremely well.

I just checked the main board one last time and measured every resistor so I'm going to install the main board tonight

Wish me luck. See you guys on the other side.
Rob.

[Enzo]

Maybe too late for you, but it is no problem to buy brass plates, here for example:

Search results for: 'brass plate'


And an alternative, most hobby stores sell brass sheet in various weights. Not particularly expensive.

[Stratz]

Thanks Enzo. While I saw your post after I installed the main board it was too late but I will try to use a brass ground plate if I ever do another Fender style build.

[Stratz]

Finished

Well..... It works guys. Sounds great and it's very quiet.

I had one issue at initial power up. The B+ was over 500V and the plate voltage was @ 90 mv with the bias pot set to minimum. Those are the quick readings that I could get before the power tubes started to run away.
I had forgotten to add one ground wire to the bias board. I added the ground and everything was fine.

The Mojo 761 (Heyboer) PT is currently putting 450V to the JJ 6V6S plates. I was concerned about this from the get go because the PT schematic shows 362v-0-362v which is 724 volts. I do have a Classic Tone 330-0-330 PT which I may install just to be on the safe side.

I originally had the JJ 6V6S's biased at about 50% PD but the breakup sounded harsh and sterile so I wound up at 19 ma with ~450v on the plates which is somewhere around 61% PD and the sound is much better. Very good in fact.

Now that the amp is finished and seems to be working very well I'd like to add reverb to the normal channel without effecting the operation in any negative way and I'd also like to use the low impedance input jack holes for 25k Mid range pots to get more mids from the amp. I think I'll start another thread for these mods though.

Attached are a few pics of the finished project minus the cabinet. It's messy but for a first build I can say that it looks cleaner than my 1973 Silver Face Deluxe Reverb did.

The terminal strip closest to the PT is split. The 4 lower tabs are for high voltage AC. I only have the AC cord ground wire and pilot light ground attached to the lower side. The upper half has the bias board and bias pot grounds, power tube pin 8 gerunds and the PT red/yellow wire and attached to it.

The two strips that run in line with the pots are all my pr-eamp grounds, etc. The outer lugs of the terminal strips (bolted to the chassis) are not connected to the rest of the inner lugs. The only lug that is fed to the chassis is the outer right lug of the strip next to the normal channel pots. Both strips are connected by the green cloth 18 ga wire.

I'd like to extend a big thank you and express my gratitude to all that were willing to share their knowledge and help me through this build. In the end it was a great learning experience. I learned a lot about how the amplifier works but still have a lot to learn before I'll know exactly what happens between the input jack to the speaker jack.

Again, many thanks to all that joined in to help me out.
Rob.

[big_teee]

High B+ seems to be the norm these days.
With higher AC main voltage being higher.
I just used a CT 40-18016 PT, on a 6V6 build.
Fender Power Transformer, Deluxe, Deluxe Reverb, Tweed Tremolux, 125P23B, 025130, 041316, 120V
It was supposed to be 330-0-330v.
It measured 354-0-354.
It produces 455-460V with a 5AR4 rectifier at idle.
So wanted to point out that the Classic Tone PTs run high also.
T

[Stratz]

High B+ seems to be the norm these days.
With higher AC main voltage being higher.
I just used a CT 40-18016 PT, on a 6V6 build.
Fender Power Transformer, Deluxe, Deluxe Reverb, Tweed Tremolux, 125P23B, 025130, 041316, 120V
It was supposed to be 330-0-330v.
It measured 354-0-354.
It produces 455-460V with a 5AR4 rectifier at idle.
So wanted to point out that the Classic Tone PTs run high also.
T

Hi T. The Classic Tone 40-18016 is the PT that I have on hand.
Lets hope I dont see more voltage from it than I do from the Heyboer (MT761) that's currently installed.
I'm using the 5AR4/GZ34 rectifier as well.....

[big_teee]

If I do anymore 6V6 builds, I'm dropping down to 295-0-295, or 300-0-300, and using diode rectifiers.
No more Rectifier tubes for me, but I realize that is the fender vintage way.
With diodes you can pretty much count on the AC voltage x 1.414.
300x1.414=424v,or 295x1.414=417v, which would be about right.
T

[Stratz]

I'd much rather run 6L6's or 5881's in the Deluxe Reverb but I just dont have the mA required with the MT/Heyboer 761 PT.

[g1]

When possible, I always try to find PT's that have a 125V primary, rather than 120V.
Our power has been above 120V for decades here.
The Hammond 'classic' series now have a 125V tap and I believe some Weber's do also.

[Justin Thomas]

My latest build has ~470/455 on plates/screens, JJ 6V6s. I blasted for about an hour the other night no problems... for what it's worth. Course, I can also plug in 6L6GCs no problem, too... I'm not so adventuresome to risk blowing any of my old 6V6s, though. JJs are cheap.

I have a 750R for shared bias, which would be about 3x the "normal" value, so in theory I should be quite cold, but I'm not noticing any sound issues, unless the Master (Crossline) is really low, which should ideally be, like, um, NEVER?

Justin

[bob p]

Excellent recommendation on the use of 125V primaries. I've been avoiding the 200 series Hammonds for decades because the 115V primary just exacerbates the problem. I always paid extra to use the 300 series instead, just to get the 120V primary tap. Now that the classic series offers 125V primaries, they're a great solution.

Regarding the ClassicTone products -- they're made locally here in Chicagoland by a division of Marvel Electric. I was talking to one of the engineers about the specs on their transformers, and he was proud to tell me that they are overbuilt and underspec'd. Well, that can be good if you're a designer who likes to use conservative ratings, but not so good if you're a builder who's trying to get predictable voltages. One of the things that I pushed for was to get them to list their secondary voltages as a function of applied load and choice of rectification. Those specs are just beginning to appear in their data sheets. If you have a specific question regarding the load conditions on a specific product, email CT and ask for details. In response to my requests for this data Mark Sacketti has started adding these specs to the data sheets. If you're interested in a transformer that doesn't have this data listed on the spec sheet, email them and ask for it. Every time I've done this they've responded by publishing an updated spec sheet with these numbers. As an example, Mark updated the data sheet for the 40-18005 in response to a request that I made over the 4th of July holiday. He responded first thing in the morning on the 5th with an updated data sheet. Customer service doesn't get better than that.