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**tbryanh** · 02-20-2009, 01:14 AM

"I was looking at the heater supply and realized that the input 12AX7 and the 6AV6 heaters were padded down to 3 volts with a couple of .68r resistors."

Datasheets show that the heater currents at 6.3v for the 12AX7 (V1) and 6AV6 (V2) tubes are 0.3 amps each. Total current through the 0.68R resistors (R35 and R36) should be 0.6 amps.

0.6 x 0.68 = 0.41v dropped across each 0.68R resistor for a total voltage drop of 0.82v.

Assuming the voltage across points W and X is 6.3v (this should be checked), then 6.3 - 0.82 = 5.46v at the heaters of V1 and V2, not 3.0.

If the heaters have only 3v, then 6.3 - 3 = 3.3v is dropped across R35 and R36. This means that each resistor has 3.3 / 2 = 1.65v across it.

1.65 / 0.68 = 2.43 amps.
1.65 x 2.43 = 4 watts

It seems likely that R35 and R36 cannot dissipate 4 watts of power and would burn up instead. (Are R35 and R35 5W resistors?) Since it was not mentioned that they were burned up, it appears the voltage measurements cannot be correct; i.e. the heater voltages at V1 and V2 with R35 and R36 un-bypassed cannot be 3v because if they were, R35 and R36 would burn up. (Unless they are 5W resistors).

Bypassing R35 and R36 may not be a problem. It appears the resistors might be surge-limiting resistors to limit the surge current through V1 and V2 during power up. Another possibility is the resistors are there to cause V1 and V2 to heat up slower during power up than V3 and V4 to prevent V1 and V2 from driving V3 and V4 until V3 and V4 have had a better chance of warming up.

**clayman** · 02-20-2009, 01:32 AM

my bogen conversion

You can see my bogen conversion in the schem.section of el34world.com
It sound great!

**tbryanh** · 02-20-2009, 04:11 AM

“I measured voltages and they are a lot higher than the schematic says. Table attached.”

The schematic shows the B+ supply to V1 as being 200v (has this been verified?) and the plate voltages as being 90v. 90v is approximately 1/2 of 200v which means V1 in the unmodified amp is biased near the middle of the swing, which sounds correct.

Apparently the cathode bias mod is causing the problem, the plate voltage is 165 or 166 instead of being about 90v.

This means the tube is biased way off center of the swing, almost to cuttoff, which is probably the reason the amp sounds so bad.

It appears that the 1K cathode resistors are not compatible with the 220K plate resistors, R7 and R9.

Some typical Fender circuits show 1500R cathode resistors with 100KR plate resistors. You might try these values.

You can leave the 220K plate resistors in if you want and decrease the values of the cathode resistors until the plate voltage gets near 90v or so.

**tbryanh** · 02-20-2009, 05:36 AM

"Regarding bias, some folks say you don't need to adhere to the 70% rule with cathode bias, what's the deal?"

I have heard that bias can be adjusted either direction from the 70% mark (within reason) to affect the tone of the amp.

"Cold" and "hot" are some of the words used to describe bias settings. I think a hotter bias means increasing to 75%, 80%, etc., and a colder bias means decreasing to 65%, 60%, etc.

**tbryanh** · 02-20-2009, 05:52 AM

It appears R21 was changed from 390R to 2.2KR. This appears to be upsetting the bias in a similar way to the bias mod that was made to V1. Consider changing R21 back to 390R.

This is probably another reason the amp sounds bad.

**tbryanh** · 02-20-2009, 06:22 AM

V2 appear to be biased incorrectly the same way V1 and V3 are, but it is not clear why.

Since B+ to V2 is 200v (has this been verified?), it appears impossible for pin 7 to have 226v on it.

It does not appear that this part of the circuit has been modified.

It appears that the voltage on pin 2 of V2 is too high, 1.47v instead of 0.8v, causing V2 to be biased near or at cuttoff.

It could be that the voltage supplying R15 is too high (greater than 270v) (has this been checked?). If not, then it could be that the value of R14 or R15 drifted. (Both values could have drifted also.)

**Enzo** · 02-20-2009, 08:39 AM

If those heaters were truly running on 3v, then perhaps one of those .68 ohm resistors was open or had gotten very high in value? Bypassing them restored the circuit. If you meant you measured 3VAC to ground from either end, that means 6v across the heaters. In any case the original circuit was not designed to put 3v on those tubes.

My guess for the resistors is to slow their warm up time a little and increase tube life considerably. These amplifiers would be installed somewhere like a restaurant and expected to run forever.

DO me a favor and see if those resistors were on value or not.

Why do you want to separate the output cathode resistor into two? In any case, you can just connect the heater CT to one of them. It isn;t a signal connection, it is just using the free DC voltage there.

R18? So lift one end of it off its connection and clip in a pot with clip wires. Now you tell us how it sounds. It is 47k now, so a 50k pot seems reasonable. But a 100k pot lets you explore even further. Then you note methodically what range of the pot is actually useful adjustment, then get a pot of that value. The 50k pot might work OK, but if you find you would never turn it over about 20k, for example, then mount a 20k pot. Or whatever.

Bias? You don;t need to adhere to the 70% rule period. It is just a rule of thumb that many subscribe to. Your power amp is only part of the whole amp system, and the only way to determine just how you want this biased is to get the amp running and find out. What sounds best in one circuit is not always what sounds best in another.

It appears R21 was changed from 390R to 2.2KR. This appears to be upsetting the bias in a similar way to the bias mod that was made to V1. Consider changing R21 back to 390R.

tbry - the 390 ohm cathode resistor didn;t work alone. There was originally the 100k R23 dawn from the 270v B+ node. He has removed that R23. They formed a voltage divider to set the bias at V3-7. All by itself, 390 ohms wouldn;t be right. He'd have to restore R23 as well. meanwhile, his V3 plates sit only 10v apart with a .1 diference at the cathodes, so I;d say it biased up fairly evenly.

This amp was working with fairly low level signals.

**Regis** · 02-20-2009, 04:05 PM

I measured 6.4 volts ac on one side of the .68 resistors and 2.99 on the other. I'll recheck it again. They look like 5 or 7 watt resistors. I'll also check their value.

The voltages in the schematic are incorrect. All the voltages in the chart are accurate. I did measure the Vs, Vp, and Va voltages but forgot to include them in the chart and now I can't find them so I'll have to remeasure them and report. The amp was recapped and the voltages are higher than what is in the schematic.

The jumper on V1 was too hard to explain schematically, it was just a wire between the pins. I wanted to be able to mix and match the two channels, it works great.

R21 was changed on advice from Enzo and MWJB to install a presence control, thread here.

Enzo I wanted to separate the cathode resistors because I have a number of unmatched single 6L6's I'd like to try. It wouldn't seem to hurt anything and each tube would run better that way? Maybe not.

**MWJB** · 02-20-2009, 04:34 PM

1K per triode is a little on the small side for V1, use one 1K & 25uf cap, shared for BOTH triodes. Even so the preamp voltage is relatively low for a typial guitar amp, changing the plate resistors for V1 and the 6AV6 to 100K will help fidelity and lift plate voltages a bit. Be aware this amp was never a super reverb.

"Regarding bias, some folks say you don't need to adhere to the 70% rule with cathode bias, what's the deal?" That's because there is no 70% rule with cathode bias. If you had fixed bias amp running 30mA @ 400v it would still sound fine, in cathode bias the amp would start to sound pretty poor, you typically need a little more current in cathode bias to get comperable clean performance. As long as you have 40+mA (20-23W per tube) you should be fine.

**tbryanh** · 02-20-2009, 08:40 PM

"I measured 6.4 volts ac on one side of the .68 resistors and 2.99 on the other. I'll recheck it again. They look like 5 or 7 watt resistors. I'll also check their value."

If after rechecking you find the voltage to be 2.99, then there is a problem. Check M2. It might be sinking current. What is M2, an indicator light?

"The voltages in the schematic are incorrect."

Were the voltages in the circuit measured before any modifications were made?

I doubt that Bogen put the wrong voltages in the schematic.

The plate voltage readings are way off of what they should be. Class A bias on preamps means the DC plate voltages should be about half of what the B+ supply voltages are, and the voltages recorded in the Bogen schematic indicate this: B+ for V1 is 200v and plate voltages on V1 are 90v, B+ for V2 is 200v and its plate voltage is 100v, and B+ for V3 is 270v and its plate voltages are 120v and 125v.

-Bryan

**MWJB** · 02-23-2009, 12:57 PM

Bryan wrote: "I doubt that Bogen put the wrong voltages in the schematic." Voltages were probably correct at the time that Bogen issued the schem, but wall voltages have risen since then so voltages noted will just be a rough guide (as they are for all 50's & 60's schematics).

Plus Regis has converted the inputs to cathode biased, rather than the original grid-leak, which will also have affected voltages.

Plate voltages of 90-125v will typically be rather low for most guitarists, so increasing cathode value at the inputs would be a good idea, perhaps reducing plate resistor value to 100K per triode too, then see what you have. If preamp plate voltages need to come up further then reduce the value of the preamp power supply dropping resistor in the B+ rail.

In most designs a 12AX7 will run more like 2/3 of the B+ voltage applied to the plate resistor. 12AY7 typically run nearer half.

**Regis** · 02-26-2009, 01:06 AM

I've been offline since last Saturday, I got spanked pretty hard by a horrific virus and had to rebuild my computer, Windows and all. I've done more changes on the Bogen and will report in a couple of days, it's sounding much better.

Thanks to all for all the help.

regis

**tbryanh** · 03-02-2009, 05:12 AM

Consider setting up your Bogen amp to be similar to the Fender Deluxe AA763 amp. (Ignore the Vibrato stuff; i.e. only use the Normal path, driver, and power amp stages as guidelines.)

The AA763 has similar voltages and numbers of stages as the Bogen (2 preamp stages, a driver stage, and a power amp stage), and the component values in the AA763 should be helpful for you to make your Bogen amp sound good.

-Bryan

Attached Files

**Regis** · 03-04-2009, 06:05 AM

Ok, I'm back. my computer was sick and so was I; I spent a week on my back with a bad cold.

Here are the additional changes I've made:

Lowered the plate resistors on the V1 and V3 to 100k to bump the voltage.
Lowered the midrange resistor to 25k to bump the midrange.
Added a 250P bright cap on the Channel 2 volume pot.
Pulled those .68 resistors in the heater circuit: One measured 4 ohms, the other was 24 ohms!!! Sheesh!
Changed the volume pot on Channel 1 to 1meg, not much of a difference so I didn't do Channel 2.
Inputs converted from grid leak bias to Fender style 33k/1meg cathode bias with shared 1k cathode resistor and cap.
Added 1/4 inch speakers jacks wired like a Fender amp.
Separate bias resistors and cathode caps for the 6l6's: 680R and 25/150. Bias current 52MA on each tube with a 5U4.
500R screen resistors added.
Removed R34.
Removed C17.
Removed R31.
82k NFB resistor was actually 100k, added parallel 68k.

I am going to add 1500r grid stoppers on the power tubes too but need to get new .033 caps first because they are little leaky.

I did another chart with voltages, see attached. Also the modded and original schematic, and a complete list of changes to the amp. I wasnt' sure of the supply voltage designations (Va, Vs, etc) so I put them in red on the modded schematic. Also, this CHA33 only has one rectifer tube so I modded the schematic accordingly.

The amp is now sounding good, kinda tweedy and brown. Funny tho, it doesn't really break up until you get close to diming it. The biggest noticable changes were the heater fix and bumping the voltages. Oh, and the midrange resistor.

Sounds good with a Tube Screamer, etc.

Kinda picking nits now, but I wouldn't mind a little more gain a little sooner. Is there anything I can change on the 6AV6 to bump it up some? Or anywhere else?

Also, what about changing the tonestack to something Fendery? I've cobbled a schematic for you to look at using the tonestack from the tweed Bassman. I modded it for a midrange resistor and not a pot because I don't have anywhere to shoehorn in another pot on the panel. I might put a switch in to go back and forth between a couple different mid resistors. Will that tonestack work or what should I do to make it work? There was a 33k and 100k resistor there that I wasn't sure if it was part of the tone stack.

More questions but that's enough for now. Thanks everybody.

I meant to take a few pics but maybe tomorrow, and at some point clips.

Regis

Attached Files