DR-Home brew Preamp Tube Plate Voltage Question.

[Mandopicker]

I have recently built a DR-style homebrew minus the normal channel and the Vibrato. Sounds great without pedals... real smooth and creamy with real nice gain characteristics, but when I kick in any type of overdrive (sparkle drive for example), the notes sound OK when chorded, but sort of grainy when riffing up the neck...hard to describe.

Anyway...this lead me to experiment with Preamp Tube Plate Voltage. With 1 k screen supply dopping reistor and a 4.7K plate dropping resistor, the voltage measured 220 on V1. Should be OK, right? But...

Looked at a Fender schematic and found that these voltages read more around 180 volts and decided to find out if this takes away of some of the grainy edge I mentioned earlier. (Figured dropping the voltage might warm or brown it up. So, leaving the Filter caps , I adjusted the resistors seperating the screen from plate supply with first using 4.7K 2 watt in both positions...then a 4.7K and a 6.8K which effectively dropped voltage in predictable incremements. I ending up with 2 x 10K 2 watt. bringing the voltage on plates to 187v...AB763 reads 180 volts V1 if I am not mistaken.

As a result, the graininess "seems" to be less...could I be on to something in an effort to change the characteristics of the preamp tube?

Cathode cap on V1 pin 3 and 8 are stock AB763 25uf/1.5 K .
All thoughts welcome.

Thanks in advance for your time.

[Amp Kat]

You shouldn't be doing anything to the screen resistors which are 470 ohms and are filtered by the choke. There are 2 10k dropping resistors between the filter caps that are the node voltages for the preamp plates. Changing the resistors will alter the voltages but I'm not sure that is your problem if you have one. Does the amp sound good clean and where is your bias set at. If it's too hot and you drive the preamp it will get real mushy on you so try backing off on the bias a tad and see what happens.

[Mandopicker]

No worries, I am familiar with the 470 ohm screen resistors and did not mess around with those...I was actually referring to tweaking the dropping resistors connected to the screen supply filter (now 10K ) and the preamp supply filter caps (now 10K) after tweaking original from values of 1k and 4.7k respectively.

The purpose of the experiment was to try and drop the preamp plate voltage to the DR schematic spec. and to see if it had an effect on the grainyness in that situation. (Using OD pedals.) My other amps, 58 Deluxe, 65 DR, 75 Champ, 73 Princeton, do not suffer from that same phenomenon. Even tried a number of different preamp tubes...though not a 12AT7...but plan to.

I built the amp with an adjustable bias, giving me the opportunity to set the exact level. There are also seperate bias test points so I can check each tube individually. I like to bias it at about 25mv per tube. (Tried it everywhere from 18 to 30 per tube prior to tweaking the voltage and this did affect tone, and to some extent the level of grainyness.)

Coincidentally, it does sound great plugged straight in with a cable...it has a real nice natural gain when pushed a bit. (More gain than my actual DR.)

You may be right...I could be listening too hard, but somehow I think it can sound better with some subtle adjustments.

Thanks for the insight...I'll mess with the bias a bit more and listen more carefully with these new voltages to hear what happens.

[chipaudette]

On my 65 DRRI, Fender put a small cap across the volume pot on the "vibrato" channel. It's similar to having a "bright" switch on your amp...but without the ability to turn it off. Since you built a 65 DR previously, you probably know the cap that I'm talking about.

The problem (in my opinion) is that "bright" cap makes it impossible to use overdrive/distortion pedals. Does your homebrew DR have this "bright" cap? If so, take it out and see if your pedals sound better.

I put my bright cap on a switch. That's what Fender did on every other model except the DR. I don't know why they had it hardwired in on this one model. Weird.

Good luck!

Chip

[Amp Kat]

You may be right Chip and are you talking about the 47pf cap ? I'd like to hear what Bruce at Mission thinks about this. Do you think leaving it out completely or changing the value and reverting it back to switchable ?

[chipaudette]

Yes, the 47 pF guy.

Chip

[Mandopicker]

Thanks for those great suggestions.

In fact, I already wired the bright cap up to a switch so I am not sure that is the cause.

Any thoughts on the effect of lower Preamp plate voltage?

Or...

The other difference is that I have been playing with my Feedback loop using a 5.6K (feedback) and 330 ohm(shunt) instead of the stock 820 (feedback) and 47 ohm (shunt) of a DR. From what I understand...the ratio is actually pretty close with both having a ratio factor of 17:1. From reading Aiken, hopefully I understand this right in that the ratio is the important measurement. (I have it hooked up to the 4 ohm speaker output tab.) That is really the most evident difference in the schematic beyond the lackof vibrato and the normal channel. (Atleast in the current version of the amp...the photos do not show the most recent changes bringing it closer to DR spec.)

Anyway...thanks again and feel free to chime in with more ideas.

Back to it!