Valvecaster / TDA2030 Combo Problems

[teej]

My wife is learning the guitar so I thought I'd build a little combo amp for her to practice with. I built the chassis out of sheet aluminum and call the whole assembly "The Noisebox." Basically, it's a Valvecaster (built from the PCB-less diagram below but laid out on perfboard) for volume and tone controls in front of a TDA2030 10-watt power amp kit from Parts Express (schematic also below) driving a 5" Celeston neodymium 4-ohm driver. The power supply (for testing purposes) is a 12 V, 750 mA wall wart from my cable modem (just to be thorough, the modem is a Motorola eXtreme SB6141).

There are no modifications or part substitutions.

The input is a 1/4" jack going into the Valvecaster. Rather than outputting to another 1/4" jack, the Valvecaster is wired to the input of the TDA2030 kit. The output from the kit goes to the speaker.

Here are schematics for the two circuits being used here. Photos are at the bottom of this thread.



Voltage Readings @ Tube Pins (neg. DMM probe @ pin 8 / vol, tone and gain pots @ full):
Pin 1 = This one was odd. The first read was 5.8 V, second was 12.35 V, and the third was 5.8 V again.
Pin 2 = -0.61 V
Pin 3 = 0.00 V
Pin 4 = 2.20 mV
Pin 5 = 12.75 V
Pin 6 = 6.65 V
Pin 7 = -0.59 V

Issue #1: When I flip on the power, it appears the whole circuit "pulses" 3 or 4 times at roughly 1 second intervals. The power LEDs (one internally on the power amp, one externally on the chassis) cut on and off and I get "pops" from the speaker at the same time and interval.

Issue #2: I get a lot of noise when the volume is at zero but as soon as I turn it up just a little, the noise decreases dramatically.

Issue #3: I'm not getting near the volume I feel I should. I have to have the volume and gain at full in order to really get good levels from the amp.

Issue #4: After watching a number of YouTube videos of this pedal in action (http://www.youtube.com/watch?v=3cMqYlhSQ8c) prior to building the Valvecaster, I was really let down when I didn't get any of the meaty, crunchy distorted goodness that I heard. My wife loves hard rock and metal, so I thought the Valvecaster would be perfect -- volume control, simple tone control and "integrated" distortion/overdrive effect when she wants it. It seems what I have here, however, is just a glorified tube-driven volume booster.

[duck_arse]

what value did you use for the volume pot, 10k or 100k? 5V8 is the more right value for pin 1, if it is 12V, it must be shorting somewhere. and all that wire is asking for trubble. the heater wires should probably be twisted together, and the valve socket should be fitted to the board, if possible. that close is optimal, your wires might be making an oscillator.

measure the power supply at switch on. is it steady, does it drop? by how much?

some experts should be along soon to help.

[teej]

what value did you use for the volume pot, 10k or 100k? 5V8 is the more right value for pin 1, if it is 12V, it must be shorting somewhere. and all that wire is asking for trubble. the heater wires should probably be twisted together, and the valve socket should be fitted to the board, if possible. that close is optimal, your wires might be making an oscillator.

measure the power supply at switch on. is it steady, does it drop? by how much?

some experts should be along soon to help.

The reading at the switch was a very steady 12 volts. And unless I misread and Parts Express sent the wrong part, I'm 99% sure the volume pot is 100k.

EDIT: The power amp kit did not come with a volume pot nor were there holes drilled or anything marked for R2 (see image below - my kit amp is identical), therefore a volume pot was not installed on the power amp. All of the controls belong to the Valvecaster. I'm not sure if this is how it should be done.

As for the wiring, the reason I didn't wire the tube to the board is because I originally used a different TDA2030 circuit (from Guitar Gadgets - abandoned it when I realized it needed more than 12 V) on a perf board that occupied the entire chassis (which measures 2" H x 9" W x 6" D) and had the tube mounted upside down from the chassis (that black thing it's mounted on in the photo is the chassis bottom). The TDA2030 kit is on a MUCH smaller PCB, so I think I'll mount the 12AU7 sideways next to the long, black C-shaped wire connecting the sleeves of the input and output.

[teej]

I started on a simplified layout last night. Here's what I've got so far.

[duck_arse]

the 10k pot on the power amp would only be loading the 100k pot on the valvecaster, so omitting it is fine.

[teej]

That's what I figured, that with the power amp's 10k pot omitted, it be at full volume all the time and controlled by the Valvecaster's volume.

Also, I double checked the value of the VC's pots last night -- they're correct. Actually, now that I think about it and visualize it, I believe the pots on the ends (volume and gain) measured the same and have the same numbering on the back which means I may have a 50k pot for the tone and 100k for gain. I'll verify this tonight.

[PRR]

The tube preamp is made for Guitar Amp input; over 100K input impedance.

This chip power amp and trimmings throws a 6K load on the preamp.

Designed for 100+K, loaded with 6K.... yeah, it'll suck.

Rip-off power amp pot R2 and jumper pin 1 to 2 (the un-groundy pins).

Remove R5 22K and replace with 220K. Data-sheet shows 100K, and fine print numbers say that 220K will cause negligible offset.

> odd. The first read was 5.8 V, second was 12.35 V, and the third was 5.8 V again.

Suggests a bad connection. 12.35V on a 12V supply means that tube is not flowing any juice at all! ~~6V is probably right. But why 6V sometimes and 12V other times? Gremlins? Bad joints?

[teej]

The tube preamp is made for Guitar Amp input; over 100K input impedance.

So should the amp be in front of the Valvecaster?

This chip power amp and trimmings throws a 6K load on the preamp.

Designed for 100+K, loaded with 6K.... yeah, it'll suck.

Rip-off power amp pot R2 and jumper pin 1 to 2 (the un-groundy pins).

There's no pot R2. The schematic shows that there should be but the PCB wasn't drilled or marked for one.

Remove R5 22K and replace with 220K. Data-sheet shows 100K, and fine print numbers say that 220K will cause negligible offset.

Pardon my lack of knowledgeable here -- basic high school electronics -- but what does increasing the resistance of R5 do for this circuit? If the datasheet shows 100k, should I use 100k instead of 220k?

I worked on simplifying my Valvecaster circuit before work this morning. All the components are in place so all that's left now is to wire the pots, jacks, etc. I also re-verified the value of those pots and yep... I had them swapped (100k for gain instead of tone).

[duck_arse]

I told you the experts would come.

the valvecaster has a v. high input impedance. show this to your guitar, keep it happy. the guitar amp part has an input impedace of ~22k, set by the value of R5, as PRR notes. changing up to 220k means the guit amp will present ten times less load on the valvecaster, keeping it happy as well.

in a perfect world, the valvecaster would have a (high Zin, low Zout) buffer stage at its output, then whatever followed it would see a low source impedance. everybody wins.

[teej]

I told you the experts would come.

the valvecaster has a v. high input impedance. show this to your guitar, keep it happy. the guitar amp part has an input impedace of ~22k, set by the value of R5, as PRR notes. changing up to 220k means the guit amp will present ten times less load on the valvecaster, keeping it happy as well.

in a perfect world, the valvecaster would have a (high Zin, low Zout) buffer stage at its output, then whatever followed it would see a low source impedance. everybody wins.

Ah, so the VC is designed to receive over 100k (as PRR said) but it's only getting 6k from the amp. The amp is after the VC, though (guitar > Valvecaster > TDA2030 amp > speaker). I'll have to break out my old electronics textbook and brush up.

[duck_arse]

you can also search this site, there is a couple of threads (in the past 6 months) both RG and PRR take to explaining the ins and outs and whens of impedance matching or not.

[teej]

in a perfect world, the valvecaster would have a (high Zin, low Zout) buffer stage at its output, then whatever followed it would see a low source impedance. everybody wins.

Might it be beneficial then to wire up such a buffer?

[duck_arse]

no. the valvecaster is fine enough if the impedance it looks into is high enough. changing the input impedance of the following amp higher, in this case, does the job. it's just the valvecaster, as a stand-alone pedal, can be plugged into anything at all, whether the input is "good" or "bad", Z-wise.

[teej]

no. the valvecaster is fine enough if the impedance it looks into is high enough. changing the input impedance of the following amp higher, in this case, does the job. it's just the valvecaster, as a stand-alone pedal, can be plugged into anything at all, whether the input is "good" or "bad", Z-wise.

Gotcha. So as a stand-alone pedal, the VC is fine as is but since I'm using it as a preamp, mods have to be made. I'll swap out that resistor in the TDA2030 amp and hopefully have it all wired and going tonight.

[PRR]

Valvecaster to 2030 (the other way makes no sense-- VC certainly won't drive your speaker).

This 2030 amp is aimed at hi-fi uses where preamps can drive 1K loads, so they used 10K pot and 22K bias resistor. (10K||22K= 6K)

The Valvecaster would normally plug into your Fender/Peavey guitar-amp, and g-amps have high impedance inputs (over 100K).

While we could patch it up with a buffer, there's really NO reason the 2030 amp HAS to use such low-value pot/resistors. In fact the pot is useless (duplicates the VC's output volume pot). The 22K resistor was an arbitrary design choice, 100K or 220K will work just as well.

So yank one pot, jumper two pins, yank one 22K and replace with 220K. Done, and a lot less pain and spouse-frustration than whomping up a buffer while your wife is hot to go.

[teej]

Valvecaster to 2030 (the other way makes no sense-- VC certainly won't drive your speaker).

That's what I meant and how it's currently wired up. Guitar plugs into VC (built into the combo) which then feeds the signal to the amp which outputs to the speaker.

This 2030 amp is aimed at hi-fi uses where preamps can drive 1K loads, so they used 10K pot and 22K bias resistor. (10K||22K= 6K)

The Valvecaster would normally plug into your Fender/Peavey guitar-amp, and g-amps have high impedance inputs (over 100K). While we could patch it up with a buffer, there's really NO reason the 2030 amp HAS to use such low-value pot/resistors. In fact the pot is useless (duplicates the VC's output volume pot). The 22K resistor was an arbitrary design choice, 100K or 220K will work just as well. So yank one pot, jumper two pins, yank one 22K and replace with 220K. Done, and a lot less pain and spouse-frustration than whomping up a buffer while your wife is hot to go.

I worked on it a little more this morning. I'm slowly getting all of the wiring together. I'm trying to keep it as simple and minimal as I can. Here's what I have so far:

[teej]

I swapped out R5 on the amp with a 220k.

Also, I've been wondering what results I would get if I swapped the TDA2030 chip with either a TDA2040 or LM1875 (I have one of both in my spare parts bin).

[PRR]

I don't even see your TDA2030, or its esential heatsink. Is it on the other side?

> what results I would get if I swapped the TDA2030 chip with either a ...

I don't know if they even have the same pinout. If not, something will die. If they are 100% the same pinout, they will give 99% the same performance. Chip power amplifiers are a mature technology. The different part numbers have different MAXimum ratings, but I think you are well below the limit on any.

TDA2030 is 36V max, suggested max is 28V total for 12W out.
TDA2030 is 40V max, suggested max is 34V total for 25W out.

However your "power supply ...is a 12 V". A 4 Ohm load on a 12V power supply can take about 4 Watts Sine power (about 2 Watts in 8 Ohms). That is what you will get. Either chip. (Any chip better than the puny LM386, which dislikes over 9V or under 16 Ohms.)

4 Watts aint stale toast. The classic Fender Champ is 5.6 Watts and many thousands of musicians learned thir chops and even recorded on a Champ. It is often too much to use at home because the wife complains. In this case, the husband may be yelling "turn it down".

Much depends on the speaker. Put a Champ into a 4-inch car speaker, it won't shout. Champ into Full Stack can be LOUD.

[bluebunny]

I don't even see your TDA2030

Removed?

because the wife complains

[teej]

I don't even see your TDA2030

Removed?

because the wife complains

Hahaha... The kits were only $10 so I bought two. In previous photos, you saw what we'll call Amp A. I left it stock and swapped the resistor in Amp B, which doesn't have the chip or heatsink installed yet. Amp B is the one in the latest photo.