Ugly, but versatile, new amp

[tubelab.com]

When I started making TubelabSE amplifiers I realized that testing newly built PC boards was a pain. I would drag out a set of transformers, tack solder the wires to the PC board, then test everything, only to disconnect the transformers and put them away. People assembling amplifiers also told me that they would like a better way to hook up the transformers. I decided to use connectors on my latest design. That worked out well so far.

I am always trying experiments with different transformers, tubes, chokes and so forth. The connectors made it easier to do this. I have ordered a batch of PC boards for this amp and I would like to have a way to test a newly built board without the setup time. I started thinking............

Then I thought of a way that I could test a board, or an OPT, or a power transformer, or choke, with only a screwdriver. So, I built it!

It is ugly, so call it a breadboard, or a test fixture. The pins that are used to plug into the screw terminals on the PC board were saved (for a long time) from my earlier life making SS-50 buss computers. The "euro - style" terminal strips are from All Electronics (also available at Radio Shack). Each OPT and the power transformer are connected using one of these.

It is not even finished yet, and I have already cranked through about 5 different sets of OPT's and 3 power transformers. I have several more waiting to be tested. After playing with it this weekend, I added the switch panel. It is not all connected up yet, but I will be able to switch from UL to triode, turn on or off the cathode feedback, switch from tube to SS rectifier, and go into standby.

[tubelab.com]

While playing with this amp, and testing the different output transformers, I captured this bass guitar run on my storage scope. This is a burst of 5 notes, each at about 90 HZ. The scope is set at 5 volts per division and connected directly across the left channel speaker. There are clean waves of about 35 volts peak to peak, across an 8 ohm (nominal) speaker. This is from an EL-34 in triode mode. Cathode feedback is on.

[arnoldc]

You call that ugly? You've got one of the neatest, coolest prototyping rig!

Let me show you what ugly means...

[tubelab.com]

Same trace, zoomed in to 10 mS per division.

I will have more pictures and test data in the next few days (time permitting) I plan to test several different OPT's and plot frequency response on them.

I am planning to order a pair of new OPT's that support UL mode. The only ones that I currently have are the small Edcors. I was planning to get a pair of the larger Edcors, but I may go with a pair of Hammond 1628SE's just because of the multiple secondary taps. This would allow me to test different tubes under different loads.

OH, the test sgnal is a CD. Gypsy Soul, new Flamenco. It is a Narada sampler CD of various "new styles of Flamenco guitar" The track used is #11. It contains a bass guitar solo that will find the limits of your amp. Later in the same track the bass is played over the Spanish guitar. Can you say intermodulation test!

[tubelab.com]

Let me show you what ugly means...

That is not even close to ugly! I have seen (and built) far worse. If we need to have an ugly amp contest, I need to make a trip to the warehouse to visit the "amps that didn't make it corner". Everyone has built one (or more) of those amps that we don't talk about on the forums. You know, the one that you hastilly put together, that never worked, or smoked on initial power up. I have a few of those, that I rob for parts now and then.

[Giaime]

Originally posted by tubelab.com
There are clean waves of about 35 volts peak to peak, across an 8 ohm (nominal) speaker. This is from an EL-34 in triode mode. Cathode feedback is on.

20W from an EL34 in triode mode single ended?

[pedroskova]

I've never been able to find ratings on the "euro-style" connectors that are found on the web. Anybody know their DCV ratings?

[Giaime]

Usually that's written under. I have here one about 20mm long that is 380V rated, 17mm ones are 250VDC rated IIRC.

Also 30mm ones are 500VDC, but that can depend on the material.

And always IIRC, laws about electrical safety impone that the voltage rating should be written on the connector.

[tubelab.com]

20W from an EL34 in triode mode single ended?

35 volts P-P across an 8 ohm resistor would be almost 20 watts. I know my speaker is NOT 8 ohms at 90 HZ, although I don't know what it really is.

I have not taken the time to accurately measure the power under the conditions that I am currently running the amp. I would suspect power near 10 watts since I am running 480 volts B+ and a 3000 ohm load. The cathode feedback allows me to lower the load impedance without sounding gross, and the high B+ gives a lot of headroom.

I am running 60 mA of tube current with 435 volts across the tube. 480 volts - 33 (cathode resistor) - 12 (OPT) = 435 volts. This gives a total tube dissipation (plate and screen) of 26 watts. Near max, but the JJ EL-34's show no signs of distress. This was not the case with the "Winged C's".

This combination sounds excellent on real music, but I haven't made measurements yet. Real measured data under different load conditions with several different transformers will follow shortly. That is one of the reasons for building this amp.


The connectors that I am using are marked 450 volts right on the terminals. It is not clear what the conditions are (AC, DC). I have the wiring arranged such that no two adjacent pins have more than 350 volts across them. I have used these connectors in my P-P breasboard with 550 volts across them. If they are going to fail, I want to find out here under controlled conditions (with a fire extinguisher nearby). I would imagine that the worst case test would be mounted to a grounded metal chassis, which is not the case in either of my breadboards.

[alexmoose]

Originally posted by arnoldc
You call that ugly? You've got one of the neatest, coolest prototyping rig!

Let me show you what ugly means...

Hey, is that an El34 PP amp? I think that is going to be my next project, so could you send me the schem, and some info about it?


Tubelab,

is the september date on the SimpleSE boards still practical?

also

could you post a full-res schematic of the Simple SE?