Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: labb on June 22, 2014, 08:36:26 am
-
Can I mount the PT for the reverb unit outside(top of) of the chassis instead of inside as the unit was originally built. I am using a chassis that is 2 x 16 x 8 and it is not deep enough to mount the transformer inside. What I a concerned about is is the tank picking up transformer noise. I intend to put this in a cab that is 19 x 8 1/2 x 9 1/2 so the tank and the transformer will be pretty close..I am trying to match the reverb unit up to a 5F6A that has the same dimensions. Thanks for any help.
-
Can I mount the PT for the reverb unit outside(top of) of the chassis instead of inside as the unit was originally built.
Probably. Mount the tank with the input jack near the PT. Here's a pic of a popular amp with the tank mounted very close to the PT.
(http://home.comcast.net/~seluckey/amps/sunn/sunn_02.jpg)
-
And here's a page with a 6G15 copy with the PT outside the chassis (http://campbellsound.com/reverbpage.html).
-
Have you considered building the Revibe unit? You have the real estate to do so. For the cost of two little tubes and a handful of components you can add a very nice harmonic vibrator circuit to the 6G15. Just an idea.
-
Thanks for the replies. As for the revibe unit, I am going to go ahead and install the extra tube sockets and take a look at adding trem at a later date. I want to finish the reverb unit and get all of the bugs out of it first. I am concerned about ground loops. (I tend to over think some things)
-
As for the revibe unit, I am going to go ahead and install the extra tube sockets and take a look at adding trem at a later date. I want to finish the reverb unit and get all of the bugs out of it first. I am concerned about ground loops. (I tend to over think some things)
Look at this from Sluckey, no ground loops with his layout;
http://home.comcast.net/~seluckey/amps/revibe/revibe.htm (http://home.comcast.net/~seluckey/amps/revibe/revibe.htm)
Brad :icon_biggrin:
-
Dang Sluckey that is really top notch work. I have built a lot of amps but I can't even come close to that kind of work. Hats off to you and thanks for the help. I notice that for the reverb section you have the Power supply ground (close to the xformer) separate from the other grounds(at the input). That is the way I normally ground. I get some push back form some builders that say that the two ground points (actually 3 if you count the mains ground) tend to give me a potential for a ground loop. The ground loop that I am actually concerned with is from connecting the reverb to the amp. Have you had any problems with this?
-
To eliminate ground loop hum you should try elevating the signal ground with 5W15R || pair of 6A diodes (nose-to-tail) || 0.1uF cap. You'll need to insulate the input and output jack grounds from the chassis and do a star ground. This way its possible to keep it ground-loop-hum-free
-
Tubeswell, that would seem to be what Fender shows with the RI unit. I just haven't figured out exactly how they are implementing it. Also don't see the cap.
-
I have not experienced any ground loop hum when connecting my Revibe between guitar and an amp. That doesn't mean that I won't have a problem at another location though. I have a ground buster AC adapter just in case I need to lift the power ground.
Here's what Hoffman had to say concerning ground loops and his Revibe unit. This info would apply to the 6G15 as well...
"I bolted the green power cord ground wire to the circuit board mounting screw in the far right hand corner of the chassis. (pots facing toward you.) Even though a 3 wire power cord is specified, you will probably have to lift the power cord ground with a wall adapter. If you do not, you may end up with a ground loop since the guitar cord that goes from the ReVibe output jack to your amp already connects both chassis grounds.
Update: I suggest you do not bolt down the green power cord wire to the chassis if your amp already has a good power cord chassis ground.
Just terminate the green power cord wire on a board lug or use some heat shrink."
-
you don't need a 5 watt resistor. 1/4 watt, MF, flameproof is all that's needed.
for point 2 point builds, tie all the grounds together to one terminal. from that terminal connect the diodes and 51R resistor to ground. tie the power entry ground to another point.
use isolated phone jacks for the input & output, and isolated RCA jacks for the input and output connections to the reverb tank.
--pete
-
Thanks Pete. I had already ordered the 15 ohm resistors, Cliff jacks and the diodes to give this system a try. It is just that I am pretty cautious when it comes to grounds certainly when it comes to mains grounds. (ie: when I don't know what the heck I am doing). So what does this network actually do? Maybe elevate the grounds by 0.7 volt.
On another topic, has anyone tried a "true bypass" switch in place of the footswitch?
I am really picking y'all's brains and I do appreciate the responses.
-
Dang Sluckey that is really top notch work.
Yes it is!
Brad :icon_biggrin:
-
So what does this network actually do? Maybe elevate the grounds by 0.7 volt.
According to Valve Wizard, the anti-phase diodes force ground current to go through the 10-15Ω resistor, where it is reduced to a negligible current.
I didn't do mine that way. I placed a 51Ω resistor in series with a 0.01uF 1kV cap, with a ground lift switch to short these out when not in the "Lift" position. In Lift, the cap disconnects the circuit from safety ground at d.c., but the values of the cap & resistor still allow RF to pass through to safety ground.
If I only use that amp and nothing else with a grounded power cord, and put the switch in "Lift" it actually causes buzzy background noise. So at least one thing in use needs to have a good connection to safety ground, if it's powered from a wall outlet.
And FWIW, I now think the issue of ground loops is extremely overstated. If the loop is 30-100ft wide traveling through power cords and outlet wiring, then okay. But inside a single amp, think you'll find over time 97% or more of hum problems (maybe even more) are caused by something other than a ground loop. Bad connection, poor solder joint, leakage in a tube, radiated hum from PT to OT, maybe radiated hum from heater wires.
-
Question for Sluckey or others: Did you connect a ground wire from the ground lug of the reverb in, reverb out and footswitch jacks to the circuit ground. Thanks for the info...I just about have this thing finished except for the cab.
-
I connected a ground wire to the reverb in/out jacks back to my filter cap buss. You can see the black wire in the pic.
I twisted a ground wire and the trem F/S wire and connected that ground to the ground buss on the board. See the blue and black twisted wire in the pic.
I didn't put a ground wire on the reverb F/S jack.
http://home.comcast.net/~seluckey/amps/revibe/revibe_03_big.jpg (http://home.comcast.net/~seluckey/amps/revibe/revibe_03_big.jpg)
-
Tubeswell, that would seem to be what Fender shows with the RI unit. I just haven't figured out exactly how they are implementing it. Also don't see the cap.
Merlin Blencowe covers this on his site (in the grounding article).
The cap is there to shunt AC out of the elevated ground return. Use isolating input and output jacks and isolating RCA jacks for the pan send return cables and have everything (jack grounds and all) wire to a single point in the signal ground return which gets isolated with the 6A nose-to-tail diodes || 15R 5W || 0.1uF. Use galactic grounding of the pre-amp ground return points with the pre-amp filter cap node etc (as you would with a normal amp ground return) - but the whole signal ground is elevated
-
More 6G15 questions. I have got the unit finished and I am concerned about the voltages I am getting. I am using a Weber WRVBPT power transformer. It is rated for 260 volt on the secondaries. I am using a BR310 Bridge rectifier. Voltages are as follows:
356 vdc at first filter cap
352 vdc at second filter cap after choke
297 vdc at third filter cap
6K6
Pin 3=345 vdc
Pin 4= 355 vdc (higher voltage on screen than on plate)
pin 8= 30.7 ( gives plate dissipation of 9.7 watt which is over the 8.5 for the 6K6)
12AX7
Pin 1= 196
Pin 6= 300
Pin 7= 135
12AT7
Pin 1= 144
Pin 6= 146
I changed out the 6K6 to a 6V6 and get a plate dissipation of 8.4 (28.6 cathode voltage). The readings put me right at the high side of the +/- 20 percent that Fender specs. Guess I am ok as long as I stay with the 6V6. What are your thoughts.
-
How does it sound? I would not worry about Fender's voltages since you used a different PT. I think your voltages are fine. I used that same PT in my Revibe. My voltages are slightly lower than yours due to the load of the two extra tubes. You can compare my voltages to yours if you want. The reverb circuit is the same as the 6G15.
http://home.comcast.net/~seluckey/amps/revibe/revibe.pdf (http://home.comcast.net/~seluckey/amps/revibe/revibe.pdf)
-
This project is terrific! Thanks a lot for sharing Sluckey! I will soon build myself a JTM45/100 and it is about the right unit I need to launch that Marshall into a whole other league. Have some of you tried the Mercury Fender Reverb unit transformers/choke for the Revibe?
Now I need to put the BOM together... I've never turreted any board. Is that hard to do? Will have to buy a step drill as well for the chassis...
Hey, I've just noticed that Doug sells a "Revibe" turreted board here! Great! Sluckey I'm in the EU and have found a chassis that would fit (albeit a bit smaller) but would you mind sharing where you bought yours? Thanks!
-
Thanks for the feedback Sluckey. To me it sounds really good. I guess I'm going to have to confess that I don't play any musical instruments. That is shame on me. I know about three or four cords that let me test them, that or I inject a cd player into them. I build them for my Son. The Reverb unit is being built to become a matched set with a 59 Bassman (5F6-A) that I built for him. The cabs will be almost identical. When I get the cab done I will post a picture of the Reverb unit and the Amp. No gut shots. Your work makes me look too bad. Wednesday I will take it to a Rock band that I know and let them try it out while they practice. Should know more then. Again, thanks to all for the help and information.
Note to Sleepless: Doug has a 6G15 BOM on the EL34 World site. Go there, go to library of information, go to board building and parts go down to you find the 6G15 board.
-
I build them for my son.
And that's a great reason to do this stuff! I'm more interested from an electronics standpoint, but I do get a lot of enjoyment picking at my guitar too. The revibe is probably my most favorite project, especially from a sound good and useful standpoint. The reverb is the best I've ever heard. I know your son will appreciate and enjoy it.
-
would you mind sharing where you bought yours?
A forum member built a couple of those chassis for me. Haven't seen him around here for several years though.
-
would you mind sharing where you bought yours?
A forum member built a couple of those chassis for me. Haven't seen him around here for several years though.
That could have been Bryan at ddawgamps.com. If so he is not in operation right now. His shop burned and he has not rebuilt.
-
EKDENTON made my revibe and warbler chassis.
-
Sluckey,
Do you think that the ON/OFF switch and power indicator could be succesfully moved to the front panel? I have my cart almost ready to pull the trigger. It is good that Hoffman sells everything you need to build this unit, except the chassis of course. He even has the choke, OT and PT. The only difference I have spotted thus far is that you use two 100uF/100uF cap cans whereas he seems to use axial caps instead. Plus a few other caps/resistor tweaks. I hope his Revibe turreted board fits your mods and everything well. I'll keep you posted on this build!
-
Do you think that the ON/OFF switch and power indicator could be succesfully moved to the front panel?
Yes. Just measure twice, drill once.
If you will be using Doug's board, I urge you to follow his layout/schematic for all interconnections. Don't use my board layout or schematic. There are several small changes in my schematic and of course my board is nothing like Doug's. You will likely get into trouble if you jump back and forth between my drawings and Doug's drawings.
Once his circuit is working, then tackle the mods I did to the tremolo circuit. I would not advise rearranging the power supply nodes on his board. Since you mentioned a smaller chassis, I'm assuming your reverb tank will not be mounted on the chassis like I did? The reverb tank is 16.75" long.
Good luck and please start a thread about your revibe project. We like to see pics too. BTW, lego4040 has a current thread about his revibe project.
-
I will for sure start a thread. I'm investigating on transformers as well. The Mojo779 PT is actually a Gibson GA5 PT without the 5V winding so that's good to know, Mercury magnetics does one too. I will try and build the board myself since I'd like to follow your layout Sluckey (no disrespect for Doug, he's the king). I need to find a blank board seller now...
The chassis I have found is just 5mm (sorry, metric...) shorter in width than yours so I figure it's no big deal. It's actually a bit longer than yours and a tidbit higher as well. For EU customers, you'll find it here, I think it would be perfect for this build:
http://www.tube-town.net/ttstore/Chassis/TT-Chassis-Metal/TT-Chassis-Alu-Typ-009-480-x-160-x-65::4809.html (http://www.tube-town.net/ttstore/Chassis/TT-Chassis-Metal/TT-Chassis-Alu-Typ-009-480-x-160-x-65::4809.html)
-
That looks like a good choice for a chassis. Good price too. All my mods are already incorporated into my board layout so just go for it. Building my board is pretty straightforward. It just requires some patience. A drill press (I use a bench top model) makes swaging the turrets really easy. Doug has a picture tutorial on his site.
Looking forward to following your progress. Lego will be happy to see another one also.