1n4007 spike protection

[lowell]

are you sure you blew the OT? To be sure you don't throw out a good OT and just FYI, maybe you already know this, but just to be careful - one side of the power tubes can redplate from other failed components and is NOT necessarily the OT.

[daz]

One side of the primary reads open, and that side read 5 volts when i checked it in the amp. I then removed all wires and checked each side of the primary against the center and one side read and the other was totally open.

[lowell]

so the resistance from center tap to one side on the primary is open AND all leads are disconnected? Be sure you have your meter set to the right range... if it's too big of a range it can read open when it's really 0-100k ohms or so. But yeah if you're measurements're accurate sounds unfortunate.

[daz]

Yep...primary center to one side reads something like 380 ohms, and the other side (the side that 5 volts in the amp) was open. The meter is autoranging.

[lowell]

darn, i'm no expert but sounds like you need to replace it.

[daz]

Yeah, but i'm not going to. I'm gonna start a new 4x6V6 amp and shelve the 18. Maybe someday later.........either that or use it for parts.

[lowell]

nice! i've built 2 5E3X2 amps.... sounds great! I really like the 2nd one cause I used a 5y3 rectifier and a 12ay7 in the first preamp position. good luck!

[daz]

Hey Lowell, if you don't mind could you show me a schematic or describe the details of the PA on your 4x6V6 amp? things like plate V, cathode or fixed, and if cathode what value R and did you use one R per pair? Also, screen R value(s) Etc etc. Anything you can tell me would help because i'm sorta flying blind, and while i know what i CAN use, i don't know what will yield results that will be tonefull w/o issues. I can't easily determine whats best myself because i'm not a tech. Oh, and will that rectifier be better than a 5AR4, and why?

[lowell]

Daz,
If I were you, and this is what I did, I'd go to Tedweber.com. You can view a stock 5E3X2 schematic under the "amp kits" page. This way you can use those values as a starting point. the 5y3 rectifier I believe has the most sag of the common rectifier tube choices. I like the sound of the amp cranked, it's very squishy. the 12ay7 preamp tube also lends a more mellow tone and less gain than the 12ax7. I have no screen resistors, however I plan on tweaking that part of the amp. I was told on this forum that adding screen resistors would be harder than changing the dropping resistor for the screen supply. However if this is done care must be taken to compensate for the other supplies... i.e. if you raise the screen supply resistor make sure to also increase the C+ dropping resistor value as well. If I'm off here someone please clarify.

[daz]

On that weber schematic theres something that confuses me. Every 6V6X2 schematic i've seen thats cathode biased has a 250 ohm cathode R. i would think a 4x6V6 amp would either have two, one per pair, or if theres only one i would think it would be either halved or doubled. But that shows a single 150 ohm. Any insight as to why this is and what i should do?

[lowell]

I changed my cathode resistor to around 120ohms. I think I added a 300ohm in parallel w/ the existing 150ohm. I think with 4 6V6 you can get by w/ a 15 watts total rating for the cathode resistor. If it was 4 6L6 you'd have to use two cathode resistors and caps split i.e. one per pair of tubes because they draw so much more current. I've read that you might benefit from splitting the cathode biasing arrangement to the pairs in that you can further balance the output tubes, but this may or may not be worth it to you.

[daz]

Ok, i'm a bit confused but let me ask this and it will help clear up my understanding at least on one point. If you use one R for all 4 tubes, and the value you picked was 150 ohm, what values would you use if you intend to change it to two resistors, one for each pair? The possible answers i would think are either double, IE:300 ohms each, half, or 75 ohms each, or the same, 150 ohms each.

now if the answer is the same, then i have another question.how can you use a 150 when every 6V6 amp i've seen uses a 250?

[lowell]

you will use the same value for both sides - 150ohms if that's the bias that you want. using 1 for all 4 tubes or 2 for each pair does not change the value of that resistor. Every tube still "sees" 150ohms to ground on the cathode either way. The only difference is that the resistor "sees" 4 tubes pulling current through it or 2 tubes pulling current through it. You can decrease the wattage value of the resistor if you're splitting it up. I use less than 250ohms, or 150ohms because I want my plate current to be higher and plate voltage a bit lower. Running tubes with less bias (hotter) gives earlier breakup, less highs, more tube compression etc... i'm pretty sure about the compression. I think that the "gain" of the tube decreases when the current is increased thus reducing output signal swing.

[daz]

Thats helps. But i'm still a bit confused about one more thing. generally i thought the cathode R should be pretty close to a particular optimum or if you stray too far you will either have very bad tone or redplate the tubes. For example, guys at 18 watt with el84's all run the cathode R's between about 125 and 150 with few exceptions. Less than 125 is where the tubes usually start to redplate, and if you went off in the opposite direction 100 ohms to say 230, the tone would apparently be way off the mark. So they generally stay in about a 20-30 ohm window. Yet you went DOWN by 100 ohms over the typical 250 ohm R i see in other 6V6 amps. By the rules i see with the el84 amps, going down from a typical R value by 100 ohms would be redplate city before you could wink an eye. Heck, i went down by about 20 ohms once from what was considered a average value and it began redplating. So i'm pretty confused as to what i should do after hearing you're running 100 ohms under the standard value.

[Enzo]

And the plate voltages were the same? Different?