My first time building a Dumblelator into an amp so it's back to newbie status for me. Lots of dumb questions:
The "Loop" diagram has a .47uF cap from the cathode to the send pot. What kind of voltage is this cap seeing? Or better yet, what brand, type, and voltage cap should I use? Is it better to mount it on the board or with the pot?
I don't see a plate resistor for the "send" side of the circuit. Does this come directly from B+? What kind of voltage should I see here? Gary's reverb-a-loop shows 368V with 315V (100k plate resistor) on the return side. Do I just run 368V to pin 1 and mount a 100k resistor between pins 1 & 6 directly on the tube?
Are the pots grounded at the input/output jacks or is there some other ground point (to avoid a ground loop)?
Pretty much the same questions on the return side. I assume the .22uF is mounted Dumble style next to the tube and the 220k/500pF input network is mounted right on pin 7?
Any help would be appreciated. I'm trying to avoid ordering a bunch of useless parts (like I usually do).
I have one in the amp I'm building now. At the screens I have a 1k resistor going to the PI and another 1k resistor going to the D-lator circuit. That line has a filter and a 100k to the plate. There is a 10k dropping resistor to another filter and another 100k resistor to the second plate. I used a cannister 32/32 filter for both. I think I might reduce the resistance to get the plate volts up a bit.
The amp is being hacked around right now because my OD relay didn't work but you can see the layout in the picture (batteries are a little low). That .47 coupler could have been a lot smaller but I had this one in the bin. 100V should be enough. I used 3 or 5 lug terminal strips for the tie points so most of that circuit is not on the board. The two parallel rn65 resistors are the plate supply.
Even though I could only test the clean channel, it and the D-lator sounded fine. My D-lator switch is on the front and the controls are on the back so I did need to shield the return wire.
Skip
You do not have the required permissions to view the files attached to this post.
Normster wrote:I don't see a plate resistor for the "send" side of the circuit. Does this come directly from B+? What kind of voltage should I see here? Gary's reverb-a-loop shows 368V with 315V (100k plate resistor) on the return side. Do I just run 368V to pin 1 and mount a 100k resistor between pins 1 & 6 directly on the tube?
The send side is a source follower circuit. With no plate resistor, the signal at the cathode is approximately the same as at the grid (in practice, slightly lower), but with much lower impedance (compared to a regular gain stage).
The source follower acts as a buffer to ensure that the effect send can drive a low impedance line level input without degrading the signal too much.
Hey Normster!!. i have built some dumbleators...so i have some experience. The send is based around a cathode follower, the cathode is that way to get gain closed to unity. The tut 1 from O'Connor explains this in detail. Being a cathode follower there is no plate resistor, B+ around 320 volts, 'cos cathode has around 19 volts. i got one loop at 335 volts, and 219 at the plate of the return stage.
Take a look at the thread about parallel loops that i started, there is a mix loop that works great.
This loop is really transparent, i always put true bypass switches to test them, and tone change is minimun.
If you need help or wanna ask something else, fell free to PM me or drop me a mail. Good luck!!
The B+ at the cathode is around 260v in a real Dumbleator. The return amp comes first in the string and then there is a rather large dropping resistor in the string before the cathode follower. 219v sounds about right to me for the return amp.
swt wrote:Hey Normster!!. i have built some dumbleators...so i have some experience. The send is based around a cathode follower, the cathode is that way to get gain closed to unity. The tut 1 from O'Connor explains this in detail. Being a cathode follower there is no plate resistor, B+ around 320 volts, 'cos cathode has around 19 volts. i got one loop at 335 volts, and 219 at the plate of the return stage.
Take a look at the thread about parallel loops that i started, there is a mix loop that works great.
This loop is really transparent, i always put true bypass switches to test them, and tone change is minimun.
If you need help or wanna ask something else, fell free to PM me or drop me a mail. Good luck!!
I was winging it but voltages came in at 265 on the return and 251 on the send plates. There is a 10k dropping resistor between the two stages and each has a filter cap. sh
265 is not on the plate. You mean the B+. If you had 265v on the plate, you would have 365v B+, then a 10k dropping would net you far more than 251 on the cathode follower. Are you taking the B+ in parallel right from the screen caps?? You need to do that and then size your dropping resistors accordingly.
If you must, reverse the string so you can get 220v on the return amp.
Why is your B+ so low??
Luthierwnc wrote:I was winging it but voltages came in at 265 on the return and 251 on the send plates. There is a 10k dropping resistor between the two stages and each has a filter cap. sh
thanks dog!! didn't know 'bout that. Each day is a learning day in here...they always worked ok the other way too...i'll try to do it like the real thing, at least in one of my amps.
d95err wrote:The send side is a source follower circuit.
Oups, wrong. It is of course a cathode follower. A source follower is the same circuit when you use a FET transistor instead (FETs and tube triodes are very similar in many regards).
I got the board back in and am ready to tackle the D-lator supply. To recap my inadequate knowledge; the supply for the send plate comes right off the power tube screens and a goodish-sized (100k?) dropping resistor goes to the return plate.
Does the circuit need any additional filter cap(s) to keep things smooth?
